Southern Chicken Casserole

2 tablespoons unsalted butter
1 tablespoon vegetable oil
1 small onion, finely chopped
1 red bell pepper, finely chopped
1 celery rib, finely chopped
1/4 cup all-purpose flour
1 1/2 cups milk
Salt and freshly ground pepper
Tabasco
1/2 cup mayonnaise
4 cups shredded rotisserie chicken
1/2 cup finely chopped pimentos (from a 4-ounce jar)
1 sleeve Ritz crackers, crushed (4 ounces)

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.