How to Make It
Melt the butter in a medium sauce-pan. Add the leek and cook over moderate heat until lightly browned, about 6 minutes. Stir in the chili powder and cook until fragrant, about 1 minute. Add the crawfish tails with their juices and the bell pepper and season with salt. Cook until the liquid is evaporated, about 8 minutes. Let cool slightly, then refrigerate until chilled, about 1/2 hour.
In a medium bowl, combine the mayonnaise with the celery, red onion, pickle, lemon juice, garlic, paprika and celery salt. Stir in the crawfish and season with salt and cayenne. Cover and refrigerate for up to 1 day.
Toast the brioche. Top each triangle with 1 tablespoon of the crawfish salad, garnish with some of the chopped tomato and a basil sprig and serve.