Creamed Chicken & Biscuits

1/2 large onion, chopped
1 1/2 tsp butter
4 C chopped, cooked chicken
1 (10.75oz) can cream of chicken soup
1 C sour cream
1/2 C milk
1/2 C chopped pimiento
1 C shredded mild cheddar cheese, divided
6 frozen biscuits, thawed

Preheat oven to 350. Grease bottom and sides of 11x7" baking dish. Heat butter in small nonstick skillet over medium-high heat until melted. Stir in onion; saute until tender. Combine onion, chicken, soup, sour cream, milk and pimiento in medium bowl and mix well. Spoon mixture into prepared baking dish. Bake 15 minutes. Sprinkle with 3/4 cup cheddar cheese. Arrange biscuits in single layer over top. Sprinkle with remaining cheddar. Bake until biscuits are golden brown and sauce is bubbly, about 20 minutes longer. Serve immediately.