Mix together onion soup mix, milk, cream of mushroom soup, and rice in a medium bowl.
Lay chicken breasts in the bottom of a lightly greased slow cooker. Place one tablespoon margarine on each chicken breast and pour soup mixture over all. Season with salt and pepper to taste and sprinkle with grated Parmesan cheese.
Cook on Low for 8 to 10 hours, or on High for 4 to 6 hours.