How to Make It
Preheat the oven to 350°. In a large saucepan, melt the butter in the oil. Add the onion, bell pepper and celery and cook over moderately high heat, stirring, until softened, about 6 minutes. Add the flour and cook, stirring, for 2 minutes. Add the milk and whisk until combined. Simmer until thickened, about 3 minutes. Season the mixture with salt, pepper and Tabasco. Remove from the heat and stir in the mayonnaise, chicken and pimentos.
Transfer the chicken mixture to a 1 1/2-quart shallow baking dish. Scatter the crushed crackers on top. Bake for about 45 minutes, until the casserole is golden and bubbling. Let rest for 15 minutes before serving.