Chicken and Rice Casserole

I added the changes i would make to this recipe in ( )

2 tablespoons butter or vegetable oil
1 medium onion, peeled and diced
3 cups diced, cooked chicken
2 (14 ½-ounce) cans green beans (or frozen peas), drained and rinsed
1 (8-ounce) can water chestnuts, drained and chopped
1 (4-ounce) jar pimentos (not even sure what this is?)
1 (10 ¾-ounce) can condensed cream of celery soup (or cream of chicken)
1 cup mayonnaise
1 (6-ounce) box long-grain wild rice, cooked according to package
directions
1 cup grated sharp Cheddar
Pinch salt

1. Preheat oven to 350. Heat butter or oil in a small skillet over
medium heat. Add onion and sauté until translucent, about 5 minutes.
Remove from heat and transfer to a large bowl.
2. Add all remaining ingredients to bowl and mix together until
thoroughly combined.
3. Pour into a greased 3-quart casserole dish. Bake for 20 to 25 minutes
or until bubbly. Let stand for a few minutes before serving.