Fire up the toaster oven for this Southwestern supper for one; it's fuss-free from prep to clean up.
Set toaster oven to broiler setting (or heat oven broiler, with rack set 4 inches from heat). Line toaster-oven pan (or rimmed baking sheet) with aluminum foil. Toss bell pepper, onion, and oil on pan; season with salt and pepper. Broil until vegetables are crisp-tender, about 10 minutes.
Push vegetables to one side; place chicken on the other. Season chicken with cumin, salt, and pepper; sprinkle with cheese. Broil until chicken is opaque throughout and cheese is browned, 15 to 20 minutes. Serve with salsa and, if desired, tortillas.