Why Basil Walnut Pesto Works on Swordfish
Swordfish steaks are dense and mild enough to carry a strong sauce without disappearing under it. A basil walnut pesto — thicker and earthier than the traditional pine nut version — clings to the fish's firm texture and adds both richness and brightness. The walnuts bring a slight bitterness that cuts through the swordfish's natural oils, while fresh basil and garlic keep the dish from feeling heavy. You spread the pesto on the fish before cooking or finish with it after, depending on whether you want the flavors to meld into the flesh or stay sharp and fresh on top.
The technique is straightforward because swordfish forgives timing mistakes. A half-inch layer of pesto won't burn before the fish cooks through, and the pesto's fat content actually helps keep the meat moist. This is not a delicate preparation — it is a practical one that works for weeknight cooking and for feeding people who are hungry.
Key Takeaways
- Toast walnuts lightly before blending to deepen their flavor and prevent the pesto from tasting raw or muddy.
- Make the pesto while your grill or pan heats so the basil stays bright green and the garlic does not oxidize.
- Spread pesto on swordfish steaks before cooking if you want the flavors to infuse the fish, or add it after if you prefer the basil to stay fresh and sharp.
- Swordfish steaks cook best at high heat for a short time — four to six minutes per side for a one-inch steak — so the pesto will not burn.
Toasting and Blending the Walnuts
Raw walnuts taste slightly bitter and can make pesto taste flat or chalky. A light toast in a dry skillet for three to four minutes changes this completely. You want the walnuts fragrant and just beginning to darken at the edges — not brown all the way through. Shake the pan every minute so they toast evenly. Tip them onto a plate to cool while you prepare the basil and garlic.
Once cooled, pulse the walnuts in a food processor until they are broken into small pieces but not yet a paste. You are looking for texture, not smoothness. Add a small handful of fresh basil leaves (about one cup, loosely packed), two cloves of garlic minced fine, and a pinch of salt. Pulse again until the basil is chopped and the mixture begins to hold together. Drizzle in good olive oil — start with a quarter cup — while pulsing, until the pesto reaches the consistency you want. Some people prefer it thick and chunky; others like it looser. Stop before it becomes a smooth spread, or you lose the character of the walnuts.
Preparing the Swordfish Steaks
Pat your steaks dry with paper towels. Moisture on the surface prevents browning and makes the pesto slide off. If the steaks are thicker than an inch, you can score the skin side lightly with a knife in a crosshatch pattern — this helps heat penetrate and gives the pesto more surface to grip.
Season the fish with salt and pepper on both sides at least ten minutes before cooking, or right before — do not season it and then wait five minutes, which draws out moisture. If you are spreading pesto before cooking, do that now, using a thin spatula or the back of a spoon to press it onto the top side of each steak. You want a layer about a quarter-inch thick. If you prefer to add pesto after cooking, set it aside in a small bowl.
Cooking on High Heat
Heat your grill or cast-iron skillet until it is very hot — you should see smoke rising from the surface. For a grill, aim for high heat with the grates oiled. For a skillet, use a high flame and add a thin film of neutral oil (not olive oil, which smokes too easily).
Place the steaks on the hot surface pesto-side up if you applied it before cooking. Do not move them. Let them sit for four to five minutes until the bottom is golden and releases easily from the grill or pan. Flip once, and cook the second side for three to four minutes, depending on thickness. The fish is done when a fork inserted into the thickest part flakes easily and the interior is opaque. If you applied pesto before cooking, it will have darkened slightly and melded into the surface — this is correct.
If you held the pesto back, spoon a generous amount onto each steak as soon as it comes off the heat. The warmth of the fish will soften the pesto slightly and help it adhere, but the basil will stay bright and fresh-tasting.
Timing and Temperature for Different Thicknesses
| Steak Thickness | First Side (Minutes) | Second Side (Minutes) | Total Time |
|---|---|---|---|
| Three-quarters inch | 3 to 4 | 2 to 3 | 5 to 7 minutes |
| One inch | 4 to 5 | 3 to 4 | 7 to 9 minutes |
| One and a quarter inches | 5 to 6 | 4 to 5 | 9 to 11 minutes |
These times assume your heat source is genuinely hot and your steaks are at room temperature when they hit the pan or grill. Swordfish is forgiving — it does not dry out as quickly as thinner fish — but it also does not need long cooking. The goal is a golden crust and an opaque interior, not a well-done steak.
If you are cooking for a crowd, you can hold finished steaks on a warm plate loosely covered with foil while you cook the rest. They will stay warm for five to ten minutes without overcooking further.
Serving and Storing Leftover Pesto
Serve the swordfish immediately on warm plates. A squeeze of fresh lemon juice over the top brightens the pesto without overwhelming it. A simple green salad or grilled vegetables on the side lets the fish be the center of the plate.
If you have pesto left over, store it in a small jar in the refrigerator for up to four days. Press plastic wrap directly onto the surface to slow oxidation and keep the basil from turning dark. You can use leftover pesto on grilled chicken, stirred into pasta, or spread on bread. It also freezes well in ice cube trays — pop out one cube and thaw it when you want a small amount for another meal.
Frequently Asked Questions
Can I make the pesto ahead of time?
Yes, but make it no more than a few hours before cooking. Basil oxidizes and darkens over time, and the garlic becomes sharper and more aggressive. If you must make it earlier, store it with plastic wrap pressed directly on the surface and keep it cold. You can also freeze pesto in ice cube trays up to three months ahead.
What if I do not have walnuts?
Pine nuts are the traditional choice and will give you a lighter, more delicate pesto. Almonds work too, though they are milder. Sunflower seeds make a pesto that is slightly less rich but still good. Toast whichever nut or seed you use, and adjust the amount of olive oil depending on how oily your choice is.
Should I remove the skin from the swordfish before cooking?
You can cook it with the skin on or off — both work. Skin-on steaks brown more dramatically and hold their shape better. Skinless steaks cook slightly faster and are easier to eat. If you leave the skin on, score it lightly so heat penetrates and the pesto adheres better.
Can I make this pesto without garlic?
Yes, though garlic is what gives the pesto its backbone. If you are avoiding raw garlic, roast a clove or two in the oven at 400 degrees for about ten minutes until soft, then blend it in. Roasted garlic is sweeter and less sharp than raw, which changes the pesto's character but keeps it delicious.
What happens if the pesto burns on the fish?
If the pesto darkens too much before the fish is done, move the steaks to a cooler part of the grill or lower the heat slightly. You can also cover the fish loosely with foil to slow the browning while the interior finishes cooking. Slightly darkened pesto is not a problem — it tastes nutty and rich — but charred pesto tastes bitter.