Baked Stuffed Flounder

Name: Baked Stuffed Flounder
Ingredients: 2 tablespoons unsalted butter; 1 medium onion, chopped; 1/2 teaspoon kosher salt, plus extra for the sweat and for seasoning fillets; 1 clove garlic, minced; 1 (10-ounce package) frozen chopped spinach, thawed and squeezed dry; 1 lemon, zested; 1/4 teaspoon freshly ground black pepper, plus extra for seasoning fillets; 2 tablespoons chopped fresh parsley leaves; 1 cup heavy cream; 1/4 cup white wine; 10 ounces grated Cheddar; 1 1/2 to 2 pounds flounder fillets; 3 cups leftover cooked rice