Turn the oven on to 350.
Grease the bottom and sides of a baking dish that is 11x7 inches or a quiche dish that is 10 inches.
Cut the onion up.
Heat butter in a small nonstick skillet and saute until tender.
In a bowl, mix the cooked onion, soup, chicken, milk, pimiento, and mayo. Then, pour the mixture into the baking dish that has been prepared.
Bake for 15 minutes and remove from oven.
On top of the baked mixture, sprinkle 1 cup of cheese.
Set biscuits in a single layer in a pretty way, and sprinkle the last 1/2 cup of cheese on top.
Put the casserole back in the oven and bake for about 20 minutes more, or until the biscuits are golden brown and the sauce is bubbling.