Preheat oven to 350 degrees F. Spray a 9 x 13 inch casserole dish with non-stick spray.
Wash the potatoes, or peel if you like. We leave the skins on. Slice them thinly, about 1/4 inch thick. Boil for about 5 minutes to soften them, but don't overcook them! They should be soft, but not done.
While potatoes are cooking, put butter and onion in a large microwave-safe dish and cook for 2-3 minutes, until onions are soft and slightly translucent. (I use my large Pyrex measuring cup.) Add remaining ingredients, mix well, and microwave 3-4 more minutes until bubbling.
Layer potatoes and sauce mixture into greased dish, evenly coating potatoes. Top layer should be sauce. Bake for about 45 minutes until the top is browned and bubbly.
(If you're pressed for time, microwave the casserole for about 10 minutes, then stick it under the broiler to brown the top - my family can't tell the difference!).