Pre-heat oven to 400°F. Optional: lightly grease 12 muffin tins.
In a medium bowl, mix together gruyere, parmesan, thyme, salt, and garlic.
In a second large bowl, coat the sliced potatoes with melted butter (I usually just use my clean hands to make sure all the potato slices are coated as evenly as possible).
In a regular-sized (12) muffin tin, layer 4 potato slices and then approximately ½ teaspoon of the cheese mixture. Continue this pattern of 4 potato slices, then cheese, until the potatoes are taller than the tin (they will shrink when they cook). You'll have 3-4 layers total, ending with cheese on top.
Cover with foil and bake for 15 minutes. Remove foil and continue to bake for an additional 25-30 minutes until golden brown and bubbly. How thinly you sliced your potatoes and how high you stacked will affect the bake time, you want them to be fork-tender with the tops and sides golden brown. Remove potato stacks with a spoon (I "scoop" them out) and serve with additional thyme as desired.
