In 3-quart baking dish, arrange flounder fillets in single layer; season with ¼ teaspoon each salt and pepper. Top with lemon; drizzle with 3 tablespoons melted butter. Bake at 450°F 12 to 15 minutes, until fish just turns opaque in center.
Pulse Italian seasoned croutons in food processor until coarsely crushed; sprinkle over flounder. Garnish with chopped parsley.