1 cup coarse fresh breadcrumbs, preferably sourdough
Kosher salt, to taste
Freshly ground pepper, to taste
Lemon wedges (for serving)
Preheat oven to 450 F. Whisk olive 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.