1/4 cup olive oil
2 tablespoons whole grain mustard
1 garlic clove, finely grated
1 tablespoon chopped fresh thyme
1 cup coarse fresh breadcrumbs, preferably sourdough
Kosher salt, freshly ground pepper
2 1 1/2–2 pound whole branzino or trout, butterflied
Lemon wedges
Preheat oven to 450°F. Whisk oil, mustard, garlic, and thyme in a small bowl. Using your fingertips or a rubber spatula, mix in breadcrumbs, squeezing or pressing to saturate bread, until evenly coated; season with salt and pepper.
Open up fish and place, skin side down, in a large ovenproof pan or on a rimmed baking sheet; season with salt and pepper. Top with breadcrumb mixture, pressing to adhere. Roast until breadcrumbs are golden brown and fish is cooked through, 10–15 minutes.
Serve fish with lemon wedges.