This cheesy Crawfish Gratin is a wonderfully decadent treat. Crawfish Gratin Ingredients 10 tablespoons unsalted butter plus more for greasing, divided 1 medium yellow onion, diced 2 cloves garlic, minced 1 cup diced celery 1 green bell pepper, seeded and diced 2 (16-ounce) packages cooked crawfish tails ¾ cup all-purpose flour ½ cup dry white