Melt butter in microwave-safe bowl. Add cabbage and onion and microwave until both cabbage and onion are soft, about 10 to 15 minutes on high power depending on your microwave. Check cabbage after 10-15 minutes to see if it is soft yet. If not soft, microwave cabbage until it is soft and cooked. After cabbage is cooked, add salt and pepper to taste. Add more butter, if needed before serving. Serves 6-8.