A bright, fresh pesto that lets delicate flounder shine

Basil mint pesto is one of the easiest ways to turn a mild white fish into something memorable. The pesto — made by pounding fresh basil, mint, garlic, nuts, and oil together — sits on top of the cooked flounder and does the work of seasoning without overwhelming the fish's own flavor. The herbs stay bright because you make them just before serving, and the flounder cooks so quickly that it stays tender and moist.

This dish works best with flounder fillets that are roughly the same thickness, ideally between a quarter and half inch. If your fillets are thicker, you can gently pound them to even them out. The pesto can be made while the fish cooks, or up to a few hours ahead and held at room temperature.

Key Takeaways

  • Pound basil, mint, garlic, pine nuts or walnuts, and olive oil together to make a pesto that tastes fresh and herbaceous rather than heavy.
  • Cook flounder fillets in a hot pan with a little butter or oil for two to four minutes per side, depending on thickness, until the flesh turns opaque.
  • The pesto goes on the fish after cooking, so the herbs stay bright and don't break down from heat.
  • A squeeze of lemon juice in the pesto and on the finished dish ties the flavors together and cuts through the richness of the oil and nuts.

Making the pesto by hand

A mortar and pestle gives you the most control and produces a pesto with texture — the herbs stay in small pieces rather than turning into a paste. Start by crushing the garlic with a pinch of salt in the bottom of the mortar. The salt acts as an abrasive and helps break down the garlic into a rough paste. Add a small handful of basil leaves (about half a cup, loosely packed) and pound them gently against the sides of the mortar until they begin to release their oils and darken slightly.

Add a smaller handful of fresh mint leaves (about a quarter cup) and continue pounding. The mint should stay visible as flecks rather than disappear completely. Add a quarter cup of pine nuts or walnuts and pound just until they break into small pieces — you want some texture, not a smooth butter. Pour in about a third of a cup of good olive oil slowly while stirring and pounding gently to combine. Finish with the juice of half a lemon and taste. Add more salt if needed. The pesto should taste bright and herbaceous, with the mint and basil equally present.

Cooking the flounder fillets

Pat the fillets dry with a paper towel. Moisture on the surface prevents browning and makes the fish stick to the pan. Heat a large skillet over medium-high heat and add a tablespoon of butter or olive oil. When the butter foams or the oil shimmers, lay the fillets in the pan in a single layer. Do not move them. Let them cook undisturbed for two to three minutes, depending on thickness. The bottom should turn golden and opaque.

Flip each fillet gently using a thin spatula, working from one end to avoid tearing the flesh. Cook the second side for one to two minutes — it will cook faster than the first side because the pan is already hot. The fish is done when the thickest part turns opaque all the way through and flakes easily when pressed with a fork. Remove the fillets to a warm plate and spoon the pesto over the top while the fish is still hot.

Variations and additions

If you cannot find fresh mint, use all basil — the pesto will be more traditional and still excellent. If you prefer a milder garlic flavor, use half a clove instead of a whole one. Walnuts work just as well as pine nuts and cost less; they have a slightly earthier taste that pairs well with the mint.

You can add a tablespoon of grated Parmesan cheese to the pesto if you like, though it is not necessary and can make the dish feel heavier. A small pinch of red pepper flakes adds warmth without heat. If you have fresh chives or tarragon on hand, a teaspoon of either one chopped fine and stirred in at the end adds another layer of flavor.

Serving and timing

Flounder with basil mint pesto is best served immediately, while the fish is still warm and the pesto is at room temperature. The contrast between the two temperatures is part of what makes the dish work. Serve with a simple green salad dressed with lemon and oil, or with boiled new potatoes tossed in butter and salt. A wedge of lemon on the side lets people add more brightness if they want it.

If you are cooking for more than two people, make the pesto first and keep it in a small bowl at room temperature. Cook the flounder in batches if necessary rather than crowding the pan — flounder releases moisture as it cooks, and crowding the pan will steam the fish instead of searing it. Each batch should take about five minutes total. Spoon fresh pesto over each batch as it comes off the heat.

Storing pesto and leftover fish

Basil mint pesto keeps in a covered container in the refrigerator for up to three days. The color will darken and the flavor will mellow slightly, but it is still good. You can also freeze pesto in ice cube trays and thaw it as needed, though the texture becomes slightly grainy. Leftover cooked flounder can be refrigerated for up to two days and eaten cold in a salad or reheated gently in a low oven, though it is best fresh.

If you have leftover pesto and no fish to use it on, it is excellent on grilled chicken, stirred into pasta, spread on a sandwich, or served with bread and cheese. A spoonful stirred into plain yogurt makes a quick sauce for vegetables.

Frequently Asked Questions

Can I make the pesto in a food processor?

Yes, though it will be smoother and less textured than hand-pounded pesto. Pulse the garlic and salt first, then add the basil and mint in batches, pulsing between additions. Add the nuts and pulse until they are broken into small pieces. With the processor running, drizzle in the oil slowly. Finish with lemon juice and taste for salt.

What if my flounder fillets are very thin?

Thin fillets (less than a quarter inch) cook very quickly — sometimes in just one minute per side. Watch them closely and flip as soon as the bottom is opaque. Overcooked thin flounder becomes dry and falls apart. If your fillets are uneven, pound them gently between two pieces of plastic wrap to even out the thickness.

Can I use dried basil or mint instead of fresh?

Dried herbs will not give you the same bright, fresh flavor that makes this dish work. If you have no fresh herbs available, consider a different preparation for the flounder. Dried herbs are better suited to long-cooked dishes where their flavor has time to develop.

How do I know when the flounder is cooked through?

The flesh should turn from translucent to opaque white all the way through. Press the thickest part gently with a fork — it should flake easily. Flounder cooks quickly, so check it early and often. Overcooked flounder becomes dry and loses its delicate texture.