Preheat oven to 375F. Spray muffin tin (traditional 12 muffin tin) with cooking oil.

Peel potatoes/sweet potatoes and slice thinly, preferable with a mandoline. Layer slices in muffin tin (smaller in diameter slices on the bottom, until half way up the muffin tin. (approx. 4-6 slices). Sprinkle the mozzarella cheese on top of slices. Divide half the chopped rosemary and sprinkle over each stack. Layer remaining potato slices (approx. 4-6 more to each stack) . (I alternated potato/sweet potato/potato/sweet potato, but you can layer however you like)

In a microwave safe bowl, heat butter,cream, salt, garlic and pepper for 30 seconds. Pour approx. 1 tablespoon of cream mixture over each potato stack.

Cover with aluminum foil, loosely and bake for 30 min. Remove foil and add Gruyere cheese. Bake an additional 5-7 min. uncovered, until cheese is bubbly and begins to brown. If you want cheese to be extra bubbly and brown you could zap under the broiler for 2-3 min. ( I did) Remove from oven and let sit for 5-10 min. before serving. Run a knife around the sides of the stacks to help loosen and make removing from the muffin tin easier. Garnish with remaining rosemary (or thyme).