Chicken, Mushroom, Broccoli, and Rice Casserole

box of long grain & wild rice, prepared per instructions

broccoli florets ((see note))

olive oil

onion (, diced)

sliced mushrooms

stalks of celery (, diced (about 1/3 cup))

cooked chicken breasts (, cubed)

of cream of chicken soup

of cream of celery soup

light mayonnaise

Salt and pepper