- In a pan, gently sauté yellow onions, bell pepper, and celery in a tablespoon of butter until soft.
- Season crawfish tails and fat with salt and pepper evenly in a mixing bowl and stir into the pot of sautéed vegetables. Turn heat to high, continually stirring to avoid scorching.
- Add remaining butter and melt. Once butter is melted and hot, add flour slowly and stir until smooth. This mixture is the roux. Simmer on medium heat stirring regularly for 15–20 minutes until roux begins to turn beige. (If you prefer a deeper roux flavor, continue to cook roux until golden brown or even brown. Be careful not to burn vegetables or scorch the pan.)
- Add stock and stir until roux is completely dissolved. Add bay leaves, sprigs of thyme, and Tabasco.
- Bring pot up to a slow boil for 5 minutes and then reduce to a slow simmer. Cover with a lid and simmer for 20 minutes. The sauce should be thick enough to coat the back of a spoon. Adjust consistency with more stock if necessary.
- Adjust seasoning with salt and pepper and, if preferred, a splash of lemon.
- Serve over steaming Louisiana medium-grain rice and garnished with fresh sliced scallions.