What This Sauce Does and How to Build It

Garlic pesto dill tuna is a tuna dish finished with a bright, herbaceous sauce made from fresh dill, garlic, and pesto—usually a basil pesto, though you can build one from scratch or use store-bought. The sauce clings to seared or grilled tuna steaks and adds acidity, richness, and a clean herbal note that doesn't mask the fish. The combination works because dill's anise-like flavor and pesto's olive oil and garlic create a sauce that tastes fresh without being heavy, and the acid (from lemon or vinegar in the pesto) cuts through the tuna's richness.

You can make this dish two ways: sear thick tuna steaks in a hot pan and finish them with the sauce, or grill them and spoon the sauce over warm fish. Both methods take about 20 minutes from start to plate. The sauce itself takes 5 to 10 minutes if you're using store-bought pesto, or 15 minutes if you're making pesto from scratch.

Key Takeaways

  • Sear or grill tuna steaks to medium-rare (an internal temperature of 110 to 115°F at the center), then rest them for 2 to 3 minutes before plating.
  • The sauce combines fresh dill, garlic, and basil pesto with a small amount of mayonnaise or sour cream to bind it, plus lemon juice to brighten it.
  • Make the sauce while the tuna rests so it's ready to spoon over warm fish; cold sauce on hot fish is less pleasant than sauce at room temperature.
  • A 6-ounce tuna steak (about 1 to 1.5 inches thick) serves one person and takes 3 to 4 minutes per side in a hot pan.

Choosing and Preparing Your Tuna Steak

Buy tuna steaks that are at least 1 to 1.5 inches thick—thinner cuts dry out in the time it takes to sear them properly. Look for flesh that is deep red or dark pink, firm to the touch, and free of brown spots or discoloration. If the fish smells strongly of ammonia or has a mushy texture, it is not fresh enough to eat raw or rare.

Pat the tuna dry with paper towels 15 minutes before cooking. Moisture on the surface prevents a good crust and steams the fish instead of searing it. Season both sides generously with salt and pepper just before the fish hits the pan—seasoning too early draws out moisture.

Searing Tuna in a Hot Pan

Heat a cast-iron or stainless steel skillet over high heat for 3 to 4 minutes until it is very hot. You should see wisps of smoke rising from the pan. Add a high-heat oil—avocado oil, grapeseed oil, or refined vegetable oil work well—and let it heat for 30 seconds.

Place the tuna steak in the pan and do not move it. Let it sear for 3 to 4 minutes until the bottom develops a golden-brown crust. Flip once and sear the other side for 2 to 3 minutes. The fish should feel firm but still give slightly when you press it with your finger—this means the center is rare to medium-rare. Remove the tuna to a warm plate and let it rest for 2 to 3 minutes before plating.

If you prefer your tuna cooked through, sear for 4 to 5 minutes per side, but know that this will make the fish drier and less tender. Most cooks who work with good tuna prefer it rare to medium-rare because the texture stays buttery and the flavor stays clean.

Making the Garlic Pesto Dill Sauce

If you are using store-bought basil pesto, measure out 1/2 cup into a small bowl. Finely chop 2 tablespoons of fresh dill and mince 1 clove of garlic. Stir the dill and garlic into the pesto along with 2 tablespoons of mayonnaise or sour cream (this softens the pesto and makes it easier to spoon). Add the juice of half a lemon and a pinch of salt and pepper. Taste and adjust—you want the dill to be noticeable but not overwhelming, and the sauce should taste bright and a little tangy.

If you are making pesto from scratch, blend 2 cups of fresh basil leaves, 1/2 cup of olive oil, 1/4 cup of grated Parmesan cheese, 1/4 cup of pine nuts or walnuts, 2 cloves of garlic, and a pinch of salt in a food processor until the mixture is chunky but holds together. Transfer to a bowl and fold in 2 tablespoons of fresh dill, 2 tablespoons of mayonnaise or sour cream, and the juice of half a lemon. Taste and adjust for salt and acid.

Make the sauce while the tuna rests so it is ready to spoon over the warm fish. A sauce at room temperature tastes better on warm fish than a cold sauce does.

Grilling Tuna Steaks

If you prefer to grill, heat your grill to high heat and oil the grates well so the fish does not stick. Pat the tuna dry and season with salt and pepper. Place the steaks on the grill and let them sit for 3 to 4 minutes without moving them. Flip once and grill for 2 to 3 minutes on the second side. The fish should have clear grill marks and feel firm but still give slightly when pressed.

Grilled tuna often has a slightly smokier flavor than pan-seared tuna, and the exterior crust is often more pronounced. The cooking time is the same as pan-searing, and you should rest the fish for 2 to 3 minutes before plating and saucing.

Plating and Serving

Place the rested tuna steak on a warm plate. Spoon 2 to 3 tablespoons of the garlic pesto dill sauce over the top, or serve the sauce on the side if you prefer. A squeeze of fresh lemon juice over the plate brightens everything further. Serve immediately while the fish is still warm.

This dish pairs well with roasted vegetables—asparagus, green beans, or zucchini all work—or with a simple green salad dressed with lemon vinaigrette. A small portion of rice or new potatoes rounds out the plate without overwhelming the delicate fish.

Frequently Asked Questions

Can I make the sauce ahead of time?

Yes, you can make it up to 4 hours ahead and store it in the refrigerator in a covered container. Let it come to room temperature for 10 minutes before serving, or warm it gently in a small saucepan over low heat, stirring often. Cold sauce tastes flat on warm fish.

What if I don't have fresh dill?

Dried dill loses its bright flavor when cooked, so it is not a good substitute here. Fresh parsley or fresh tarragon will work in a pinch, though the flavor will be different—tarragon is more anise-forward, and parsley is more neutral. You can also omit the herb and make a simple garlic pesto with lemon.

How do I know when the tuna is done?

The most reliable way is to use an instant-read thermometer: rare is 110 to 115°F at the center, medium-rare is 115 to 125°F. If you don't have a thermometer, press the fish gently with your finger—it should feel firm on the outside but still give slightly in the center. Overcooked tuna is dry and tough.

Can I use canned tuna for this sauce?

Canned tuna is already cooked and has a different texture and flavor than fresh tuna steak. You can toss drained canned tuna with the sauce and serve it cold as a salad, but it will not have the same appeal as fresh seared tuna.