In a 2 quart casserole dish, microwave potatoes, carrots, water, butter, and instant onions on high until tender (about 10 minutes).
Mash vegetables and add salt, celery salt, pepper, and milk.
Microwave on 1/2 power for about 15 minutes.
"Later I experimented and added 1 lb of browned ground deer sausage, and we really like the spicy flavor."