I know I know, enough with the butternut squash, already! Well, it's good. And there always seems to be one somewhere in my kitchen that needs to be used up.
The key to this is to slice everything very thinly. Peel, seed and slice a butternut squash and a few potatoes. Dice a shallot, and saute that in a pan with a little olive oil. To that add some sliced mushrooms (any kind). Mix together two eggs and half a pound of goat cheese until creamy. Then layer potatoes, squash, goat cheese mix, and sauteed mushrooms, in that order. Repeat until you run out of stuff, or room in the casserole pan. Then pour heavy cream over the lot until it rises to the top of the pan. Bake at 350 until done. The original recipe called for 50 minutes, but that turned out not to be nearly enough time. I'd say at least 90 minutes, maybe more, depending on how thick the slices of potato and squash ended up being. When you can put a fork all the way the lot, it's done. Remove from the oven and let cool before slicing, so the gratin can congeal.
The most expensive thing in here is the half-pound of goat cheese (maybe the cream). Maybe three dollars' worth of ingredients total in this thing? Especially if you buy it all in bulk, like I do. There were at least 12 servings in mine.