Preheat oven to 350.
2 Grease the bottom and sides of an 11×7 inch baking dish or 10 inch quiche dish.
3 Chop the onion.
4 Heat butter in a small nonstick skillet and saute until tender.
5 Combine cooked onion, soup, chicken, milk, pimiento, and mayo in a bowl and then turn into the prepared baking dish.
6 Bake for 15 minutes and remove from oven.
7 Sprinkle 1 cup of the cheese over the baked mixture.
8 Arrange biscuits decoratively in a single layer and sprinkle remaining 1/2 cup cheese over them.
9 Return casserole to oven and bake until biscuits are golden brown and sauce is bubbly, usually about 20 minutes longer.