Preheat oven to 350°F. Spray 11x7-inch baking dish with vegetable spray. Chop onion & saute in butter until tender. Combine onion, chicken, chicken soup, sour cream, milk & pimento in bowl & mix well.
Spoon mixture into prepared baking dish & bake for 15 min. Remove from oven.
Sprinkle baked layer with 3/4 cup of cheddar cheese. Arrange biscuits in single layer over top. Put remaining cheese on top. Bake until biscuits are golden brown & the sauce is bubbly, about 20 minutes longer. Serve immediately.