A simple butter sauce that lets swordfish be itself

Rosemary dill butter is the fastest way to turn a swordfish steak into something memorable. You make a compound butter — fresh herbs mixed into softened butter — and lay a slice on top of the hot fish as it comes off the heat. The butter melts into the flesh, carrying the sharp green notes of dill and the piney warmth of rosemary without drowning the fish's own clean, meaty flavor. The whole dish takes 20 minutes from pan to plate.

Swordfish's firm, dense texture is what makes this work. Unlike delicate white fish that falls apart under sauce, swordfish holds its shape and stands up to bold flavors. The butter doesn't mask the fish — it amplifies it. You taste the swordfish first, then the herbs follow.

Key Takeaways

  • Make the compound butter ahead by mixing softened butter with fresh rosemary, fresh dill, salt, and lemon zest, then chill it until firm.
  • Swordfish steaks need high heat and a short cooking time — about 4 to 5 minutes per side for a 1-inch steak — to stay moist inside.
  • Pat the fish completely dry before it hits the pan so the surface browns instead of steaming.
  • A single slice of cold herb butter on top of the hot fish is enough; the residual heat does the melting.

Making the compound butter

Start with unsalted butter at room temperature — it should be soft enough to mix easily but not greasy or separated. Cut a stick into chunks and let it sit on the counter for 15 to 20 minutes while you prep the herbs. Finely chop fresh rosemary (about 1 tablespoon) and fresh dill (about 2 tablespoons). The pieces should be small enough that they distribute evenly through the butter without leaving big herb clumps.

Combine the softened butter with the herbs, a pinch of salt, and the zest of half a lemon. Mix gently with a fork until the herbs are evenly distributed. Lay a piece of plastic wrap on the counter, spoon the butter onto it in a rough log shape, and roll it up tightly. Twist the ends so the butter is sealed inside. Refrigerate for at least an hour — overnight is fine. When you're ready to cook, slice the log into quarter-inch rounds. You'll use one slice per steak.

If you don't have time to chill the butter, you can still make this work: spoon the herb butter directly onto the hot fish and let it melt there. The presentation is less neat, but the flavor is the same.

Preparing and cooking the swordfish

Use steaks that are about 1 inch thick — thinner and they dry out too fast, thicker and the outside burns before the inside cooks through. Pat them completely dry with paper towels. Moisture on the surface prevents browning and creates steam instead of a crust. Season both sides generously with salt and pepper just before cooking.

Heat a cast iron or stainless steel skillet over medium-high heat until it's hot enough that a drop of water sizzles and evaporates immediately. Add a thin film of neutral oil — vegetable, canola, or light olive oil. Lay the swordfish steaks in the pan without moving them. Let them cook undisturbed for 4 to 5 minutes. The bottom should develop a golden-brown crust. Flip once and cook the other side for another 4 to 5 minutes. The fish is done when a fork inserted into the thickest part meets gentle resistance — it should not flake apart easily, but it should not feel hard either.

Transfer the steaks to a warm plate. Top each one immediately with a slice of cold herb butter. The heat from the fish will melt it into the flesh. Let it rest for a minute before serving.

Why this butter works with swordfish

Rosemary and dill are both assertive herbs, but they work in different ways. Rosemary brings a resinous, almost piney heat that sits on the palate. Dill is brighter and more delicate — it has a subtle anise note that doesn't overpower. Together, they create a flavor that's complex without being heavy. The lemon zest adds acidity that cuts through the richness of the butter and keeps the dish from feeling greasy.

Swordfish's flavor is mild and slightly sweet compared to darker fish like tuna or mackerel. It needs herbs that have presence without aggression. Dill especially is traditional with fish in Scandinavian cooking for this reason — it's herbaceous without being harsh. The butter carries these flavors directly into the fish's flesh, where they belong.

Variations and what to serve alongside

If you don't have fresh dill, fresh tarragon works well — it has a similar delicate quality. Fresh chives are milder but still good. Avoid dried herbs for this butter; they don't distribute evenly and can taste dusty. If you're using what you have on hand, make sure the herbs are fresh and fragrant.

Serve the swordfish with something that won't compete: roasted lemon halves, a simple green salad dressed with vinegar, or steamed asparagus. A piece of crusty bread is useful for soaking up the melted butter. A dry white wine — Sauvignon Blanc, Pinot Grigio, or Vermentino — echoes the brightness of the dill and lemon without overwhelming the fish.

Storing and reusing leftover herb butter

Compound butter keeps in the refrigerator for up to a week and in the freezer for up to three months. If you have slices left over after cooking the swordfish, wrap them tightly and save them. They're useful on grilled chicken, roasted vegetables, or a piece of white fish. You can also melt a slice into hot pasta or rice.

If your butter has been sitting at room temperature for more than an hour, discard it. Butter with fresh herbs can develop bacteria if left unrefrigerated, especially in warm weather.

Frequently Asked Questions

Can I make the herb butter the day before?

Yes. Wrap the log tightly in plastic wrap and refrigerate overnight. The flavors actually deepen slightly as the herbs infuse into the butter. Slice it fresh when you're ready to cook.

What if my swordfish steak is thicker than an inch?

Lower the heat to medium and cook longer — about 6 to 7 minutes per side. You want the outside to brown without the inside drying out. A meat thermometer inserted into the thickest part should read 130°F for medium-rare.

Can I use frozen swordfish?

Yes, but thaw it completely in the refrigerator first — at least 8 hours. Pat it dry before cooking. Frozen fish releases more water as it thaws, so drying it well is especially important for getting a good crust.

What if I don't have a cast iron skillet?

Any heavy-bottomed pan works — stainless steel or a heavy nonstick. Avoid thin pans that heat unevenly. The goal is a surface hot enough to brown the fish quickly without burning the outside.

Can I grill the swordfish instead of pan-searing?

Yes. Heat the grill to medium-high, oil the grates, and cook the steaks for 4 to 5 minutes per side. Top with the herb butter the same way. Grilled swordfish has a slightly different texture — a little less crust — but the butter works just as well.