Heat oven to 400°F. Spray 15x10x1-inch pan with cooking spray.
Brush chicken with 2 tablespoons of the mustard, then coat with baking mix. Place 1 chicken breast in each corner of pan. Place potatoes, bell pepper and onion in center of pan; brush vegetables with remaining 2 tablespoons mustard. Spray chicken and vegetables with cooking spray; sprinkle evenly with cheese and paprika.
Bake uncovered 35 to 40 minutes, stirring vegetables after 20 minutes, until potatoes are tender and juice of chicken is clear when center of thickest part is cut (170°F).