Saute onion and mushrooms in butter in a large skillet until tender.
Add peas, bean sprouts, and water chestnuts.
Spoon mixture into a shallow, lightly greased 2-quart casserole.
Combine soup, milk, salt, and pepper, mixing well, stir into pea mixture.
Sprinkle chowmein noodles around edge of casserole.
Bake, uncovered, at 350Â° for 30 minutes.