Halibut Olympia

Whisk olive oil, dry mustard, melted butter, 1 tablespoon lemon juice, 1 tablespoon kosher salt, garlic powder, red pepper flakes, 1 teaspoon black pepper, and 1 teaspoon dried dill in a 9x13-casserole dish.

Coat fish with seasoned olive oil mixture, turning several times to coat. Cover dish with plastic wrap and refrigerate for 1 hour (up to overnight) to marinate.

Preheat oven to 350 degrees F (175 degrees C).

Mix sour cream, red onion, 3 tablespoons lemon juice, parsley, garlic, 1 tablespoon kosher salt, 1 teaspoon black pepper, and 1 teaspoon dill in a bowl until combined.

Spread sour cream mixture over fish.

Sprinkle Cheddar cheese over sour cream mixture.

Bake in the preheated oven until the fish flakes easily with a fork, about 20 minutes.

Remove fish from oven and top with French-fried onions. Return to oven and bake until onions are lightly browned, about 10 more minutes.