Preheat oven to 375°F. Spray sheet pan with nonstick cooking spray.
Arrange onion and pepper mixture in a single layer on pan.
Bake for 13 to 18 minutes.
To assemble sandwich, layer each bun with 2 ounces hot diced chicken and 1/4 cup vegetable mixture. Ladle each sandwich with 2 1/2 tablespoons Ultimate White Cheese Sauce.
CCP: Heat to 165°F for at least 15 seconds
CCP: Maintain hot food at 135°F or above