Use a mandolin to slice the potatoes evenly. For perfectly round gratins, use only the middle portion of large potatoes and save the scraps for a separate recipe.
Pre-simmer the slices in a garlicky, thyme-infused cream bath. This cooks the potatoes slightly to save time in the oven and thickens the cream so it won't curdle in the oven.
Coat the muffin tin cups with butter and fill them halfway with the simmered potato slices. Add a layer of grated Gruyere cheese, then fill the cups to the top.
Spoon in cream to cover the gratins and finish with more grated cheese. After baking, use a table knife to loosen them from the tin — they'll pop out best when slightly cooled.