5 lbs Yukon gold potatoes, cut in large chunks
8 oz sour cream
6-8 T butter
1/2 c sour cream
1/4-3/4 c milk/half-and-half/cream
12-16 oz mushrooms, cremini or white, thickly sliced
2 medium sized yellow onions, cut in half, then into 1/4" slices
Boil potatoes in salted water until a knife pierces them easily. Dump in a colander to drain, and throw the butter in the hot pot to melt. As soon as you can handle the potatoes, peel (if desired) and then add back to pot. Pour in a little of your liquid dairy and mash lightly, making sure to incorporate the butter from the bottom. Add sour cream, and more dairy as needed and mash to desired consistency (pretty smooth but not whipped). Season with salt as needed and lightly with white pepper.
While potatoes are cooking, saute mushrooms in enough olive oil to keep them from sticking, lightly salted and peppered. In a separate pan, do the same with the onions. If you cook them in the same pan, they will likely not be done at the same time.
To assemble, butter a large casserole dish and transfer half of the potatoes to it. Spread the onions on the top, then the mushrooms. Carefully put the rest of the potatoes on top of the mushrooms (they may not spread easily so use your fingers if needed). Top with shredded fontina, grated parmesan, or any other cheese that suits your fancy.
Bake covered for 40 minutes, uncover, and continue cooking until hot through and cheese is melted.
