1. Grease a large (3-quart or 8 cup) microwaveable casserole or serving dish with a little of the melt butter.
2. Arrange the potatoes and onions layers and sprinkle with salt and pepper.
3. Pour over the water and cover the dish with glass lid, plastic wrap or wax paper.
4. Place in the microwave and cook for about 9 minutes.
5. Layer corned beef and cabbage over the vegetables in the casserole.
6. Drizzle the butter evenly.
7. Recover the dish and return to the microwave and cook for another 11 minutes or so until the cabbage is tender-crisp and potatoes are fork tender.
8. Allow to stand for 5 minutes covered in the microwave before serving.
9. Take the dish straight to the table and serve hot.