Step 1

Grease 9 by 13-inch casserole dish with softened butter. Preheat oven to 400 degrees F.

Step 2

Arrange fish fillet in the prepared casserole dish. Sprinkle with salt and pepper.

Step 3

Heat 2 teaspoons oil in a medium heavy skillet over medium-high heat. Add onion and cook, stirring often (adding 1 to 2 tablespoons water if necessary to prevent the onion from darkening too much) until the onion softens and starts to brown, 6 to 9 minutes. Stir in thyme and nutmeg. Spread the onion mixture over the fish. Pour wine into the casserole dish.

Step 4

Pulse bread in food processor until coarse crumbs. Drizzle on the remaining 2 teaspoons oil and add paprika and garlic powder. Process to combine until the bread is evenly red and moist. Sprinkle crumbs over the onion.

Step 5

Top the casserole with the cheese and place in the oven. Bake until the fish is cooked through and the crumbs are browned, 18 to 22 minutes. Serve warm.