Preheat oven to 375°F. Coat a 12-cup muffin tin with cooking spray.
Alternate 20 to 24 potato slices and cheese in each cup, ending with cheese on top. Cut potatoes to fit if necessary.
Season with salt and pepper and drizzle 1 to 2 Tbsp heavy cream over each stack. Cover pan with foil and bake for 20 minutes. Remove foil and bake an additional 15 to 20 minutes. Top with green onions.