Preheat the oven to 350 degrees.
Melt the butter in a pan over medium high heat. Add the garlic and cook for a minute or so.
Add in the cream, mustard, salt, and pepper. Cook for a few minutes to let the temperature rise.
Slowly add in the shredded fontina cheese while whisking.
Arrange half of the thin potatoes in a baking dish. Pour half the sauce on, along with half of the Gruyere cheese.
Arrange the other half of potatoes on top. Pour the rest of the sauce and sprinkle the second half of the cheese on top.
Place the dish on a cookie sheet to catch any drippings while baking.
Bake for 90 minutes, or until potatoes are cooked through and cheese is bubbly.
Sprinkle some fresh thyme on top as a garnish.Enjoy!