Brown the hamburger with the onion and drain the grease. Grease a 9-inch by 13-inch pan and preheat oven to 350 degrees. Layer the bottom of the pan with the meat and onion mixture. Cover this layer with cheese.  Combine the cream of chicken soup with water (add peas here) and spread over the casserole. Top with the bag of tater tots. Bake at 350 degrees for 45 minutes or until thoroughly cooked.