Preheat oven to 325 degrees F. In a skillet saute bacon lardons until lightly crisp and brown. Remove with a slotted spoon to paper towels to drain. Discard all but 2 teaspoons bacon fat from skillet , reheat, add mushrooms and saute until lightly brown; season with salt and pepper.

Generously butter a shallow baking dish with 2 tablespoons butter. Toss potato slices with salt and pepper and arrange half in a layer in baking dish. Sprinkle with bacon and mushrooms and top with h alf of cheese. Top with a layer of remaining potatoes, and pour enough cream over top to barely cover potatoes. Dot with remaining butter. Bake 30 minutes until cream is completely absorbed and potat oes are tender. Remove baking dish from oven, sprinkle with remaining cheese and continue to bake until cheese is melted, browned and bubbly, 10 to 15 minutes more. Let sit a few moments before servi ng.