Combine mix with milk in Multipurpose Pot. Whisking constantly, bring to a boil over medium-high; reduce heat, and simmer 3 min or until thickened.
Meanwhile, place chicken in Multipurpose Steamer. Cover; microwave on high 3–5 min.
Thinly slice bell pepper and onion, if using.
Place pizza crusts on two separate Sheet Pans. Spread sauce over pizza crusts; arrange chicken and vegetables on top. Sprinkle with cheese.
Bake until cheese melts, 8–10 min. Remove from oven and sprinkle with topper, if desired. Cut each pizza into 8 slices; season to taste with salt and pepper.