etouffee recipe

Ingredients: 1/2 cup unsalted butter 1/2 cup all-purpose flour 1 large onion, finely chopped 1 green bell pepper, finely chopped 2 celery stalks, finely chopped 4 cloves garlic, minced 2 cups seafood stock (or chicken stock) 1 can (14.5 oz) diced tomatoes, drained 1 pound crawfish tails (or shrimp)