For an even deeper flavor, consider searing the shallots and bell peppers in the skillet after browning the chicken. This will add a caramelized sweetness to the casserole. Feel free to adjust the amount of cayenne pepper to control the level of heat. If you prefer a milder dish, omit it altogether. Serve this casserole with warm tortillas, rice, or a dollop of sour cream and fresh cilantro for a complete and satisfying meal.