Baked Stuffed Flounder

Ingredients

• 2 tablespoons unsalted butter
• 1 medium onion, chopped
• 2 1/2 teaspoons kosher salt, divided
• 1/2 medium shallot, minced
• 1 (10-ounce) package frozen chopped spinach, thawed and squeezed dry
• 1 lemon, zested
• 1 1/2 teaspoons freshly ground black pepper
• 2 tablespoons chopped fresh parsley leaves
• 1 cup heavy cream
• 1/4 cup white wine
• 1 tablespoon lemon juice
• 10 ounces cheddar cheese, grated
• 1 1/2 to 2 pounds flounder fillets
• 2 cups leftover cooked long grain white rice