How to Cook Swordfish with Lemon Pesto Butter

Swordfish's dense, meaty flesh holds up to bold flavors without falling apart, which is why lemon pesto butter works so well. The butter melts into the heat of the fish, the pesto stays bright instead of turning muddy, and the lemon cuts through the richness without overwhelming it. The technique is straightforward: sear the fish hard on both sides to build a crust, then finish it in the oven or under the broiler with a spoonful of pesto butter on top, letting the residual heat melt it into the flesh as it cooks through.

The key is not to overthink it. Swordfish is forgiving because it has so little fat that it won't dry out the way a leaner white fish might. You're aiming for an internal temperature of 140°F, which takes about 8 to 12 minutes depending on thickness. The butter should be soft enough to spread but still cold enough to hold its shape when you dollop it on the hot fish.

Key Takeaways

  • Pat your swordfish steaks completely dry before searing, because moisture on the surface prevents browning and creates steam instead of crust.
  • Make the pesto butter ahead by mixing softened butter with fresh basil pesto, lemon zest, and a pinch of salt, then refrigerate it until you're ready to cook.
  • Sear the fish in a hot cast-iron or stainless steel pan for 3 to 4 minutes per side to develop a golden crust before moving it to the oven.
  • A 1-inch-thick swordfish steak needs about 8 to 10 minutes in a 400°F oven after searing, and you can check doneness by pressing gently—it should feel firm but not hard.

Choosing and Preparing Your Swordfish Steaks

Buy swordfish steaks that are at least 1 inch thick, cut perpendicular to the spine so they hold together during cooking. Thinner pieces cook too fast and dry out before the pesto butter has time to melt into them. Look for flesh that is pale pink or off-white with a fine grain, not translucent or mushy. If your fishmonger cuts the steaks to order, ask them to cut them thick and ask whether the fish came in that morning or the day before.

Remove the swordfish from the refrigerator 15 minutes before you cook. While it comes to room temperature, pat it completely dry with paper towels—this is not optional. Any moisture on the surface will turn to steam and prevent the crust from forming. Salt the steaks generously on both sides about 5 minutes before they go into the pan. The salt draws a tiny amount of moisture to the surface, which then evaporates in the heat and helps the crust develop.

Making Lemon Pesto Butter

Start with 4 ounces of unsalted butter, softened to room temperature but not melting. Stir in 3 tablespoons of basil pesto (homemade or store-bought), the zest of one lemon, and a small pinch of fleur de sel or kosher salt. If your pesto is already salty, taste before adding more. The butter should be pale green and flecked with basil, not uniformly colored.

Spoon the mixture onto a piece of plastic wrap, roll it into a log about the thickness of a quarter, and refrigerate it for at least 30 minutes. You can make this up to three days ahead. When you're ready to cook, slice the log into rounds about ¼ inch thick. One round per steak is usually enough, but you can use more if you like the richness.

If you don't have time to chill the butter, you can use it soft, but it will melt faster and run off the fish instead of staying put. Cold butter also gives you more control over how much melts into the fish during cooking versus how much stays on top as a sauce.

Searing and Finishing in the Oven

Heat a cast-iron or stainless steel skillet over medium-high heat for 3 minutes. You want the pan hot enough that a drop of water sizzles and evaporates immediately, but not so hot that it smokes. Add a thin film of oil with a high smoke point—grapeseed, avocado, or refined vegetable oil all work. Swirl to coat the bottom.

Place the swordfish steaks in the pan without moving them. Let them sit for 3 to 4 minutes until the bottom is golden and releases easily from the pan. Flip once and sear the other side for another 3 to 4 minutes. The fish should have a deep golden crust on both sides. Transfer the skillet to a 400°F oven, or if your skillet handle is not oven-safe, transfer the steaks to a baking dish.

Place one round of pesto butter on top of each steak and slide the pan into the oven. Bake for 8 to 10 minutes, depending on thickness. The fish is done when it feels firm to the touch but still has a slight give—press it gently with your finger, and it should spring back slowly. An instant-read thermometer inserted into the thickest part should read 140°F.

Broiler Method for Faster Cooking

If you prefer to finish the fish under the broiler instead of in the oven, sear it the same way in a cast-iron skillet, then top with the pesto butter and slide the whole skillet under a preheated broiler set to high. Position the rack so the top of the fish is about 4 inches from the heat source. Broil for 6 to 8 minutes, watching carefully so the butter doesn't burn. The fish will cook faster this way because the heat comes from above as well as below.

The broiler method gives you a slightly more pronounced crust on top and melts the butter faster, but it requires more attention. If you step away, the butter can brown too much and turn bitter. The oven method is gentler and more forgiving, especially if you're cooking for guests and don't want to hover over the stove.

Serving and Pairing

Slide the swordfish onto a warm plate while the butter is still melting into the flesh. Spoon any pan juices over the top. A squeeze of fresh lemon juice right before eating brightens everything without making it sour. Serve with something that won't compete with the pesto and lemon—roasted asparagus, sautéed spinach, or a simple green salad dressed with just oil and vinegar all work well.

If you made extra pesto butter, you can serve it on the side for people who want more. It also keeps in the refrigerator for up to five days and can be used on other grilled fish, roasted chicken, or melted over steamed vegetables. A crisp white wine like Vermentino or Sauvignon Blanc pairs naturally with the lemon and basil, but a light rosé works too.

Frequently Asked Questions

Can I use frozen swordfish steaks?

Yes, but thaw them completely in the refrigerator before cooking—never at room temperature. Pat them very dry after thawing because frozen fish releases more water as it thaws. The cooking time will be the same as fresh once they're fully thawed.

What if I don't have fresh basil for the pesto?

Store-bought pesto works fine and is actually more reliable because it's already balanced. You can also make a quick pesto with parsley and a small amount of garlic if basil isn't available, or use a different herb butter altogether—tarragon and lemon, or dill and capers both suit swordfish.

How do I know if my swordfish is overcooked?

Overcooked swordfish becomes dry and chalky. If you cut into it and it looks opaque all the way through with no translucence at the center, it's overdone. Aim for the center to still look slightly moist and pale, not bright white. It will continue cooking slightly after you remove it from heat.

Can I cook swordfish on the grill instead?

Yes. Oil the grill grates well and preheat to medium-high heat. Sear the steaks for 3 to 4 minutes per side, then move them to a cooler part of the grill or reduce the heat, and cook for another 4 to 6 minutes until done. Top with pesto butter right before serving so it doesn't burn on the grill.

What thickness should I ask for at the fish counter?

Request steaks that are 1 to 1.5 inches thick. Anything thinner than 1 inch will cook too fast and dry out before the butter has time to flavor it. Anything thicker than 1.5 inches will take longer to cook through and risks overcooking the outside while the inside is still cool.