3 tablespoons butter, cut into cubed pieces (plus more for the bottom of the gratin dish and cooking the mushrooms and spinach)
1 1/2 pounds fingerling potatoes (or any other waxy potatoes - these work a bit better since the skins soften better in the oven)

1 cup milk

3 oz cheese parmesan, gruyere (yum), cheddar - whatever your liking! We used a healthy mixture of white cheddar and parmesan
10 oz mushrooms (we used white button, but feel free to use any other variety), de-stemmed and sliced

1 cup (or generous handful) washed spinach salt/pepper