Preheat your oven to 350°F. Peel and thinly slice (on a mandolin or by hand) about 3-4 medium potatoes.
In a medium sauce pot, add butter and melt over medium heat. Add the garlic and thyme and sauté for about 1 minute, or until fragrant. Add the flour and stir until combined. Slowly add the milk and cream whisking constantly so there are no lumps. Bring to a light boil for about 2-3 minutes and turn off heat. Add the mustard and season the sauce with salt and pepper.
Grease an ovenproof baking dish with butter or non-stick cooking spray. On the bottom layer spoon 1/2 cup of sauce and layer with sliced potatoes so they fan out and make a single layer overlapping by about half their size. Then pour 1/2-3/4 cup of sauce and sprinkle about 1/2 cup of cheese. Repeat this layering until the dish is full. On the final layer, pour on remaining sauce and sprinkle cheese. Cover with foil and bake for 1 hour covered followed by 30-45 minutes uncovered, or until the sauce is thick and reduced and the surface is golden brown. Allow to cool slightly before serving.
