Creamed chicken and biscuits casserole recipe

Chicken second courses

1 small onion 1½ teaspoons butter 2 lbs chopped cooked chicken breasts or 2 lbs boneless skinless chicken thighs 1 (10¾ ounce) can cream of chicken soup ½ cup mayonnaise 1 cup milk ½ cup chopped pimiento or ½ cup sauteed red bell peppers or ½ cup jarred roasted red pepper 1½ cups shredded mild cheddar cheese refrigerated biscuit