Scrub the potatoes, peel if desired. Thinly slice the potatoes to about ⅛-inch thickness.
Line the sliced potatoes in a greased 2.5-quart casserole dish. Standing them on edge makes for a unique presentation but don't pack them in too tightly.
In a saucepan over medium heat, melt the butter. Then sauté the onion and garlic until tender.
Stir in the sweet rice flour and cook for 2 minutes.
Gradually stir in the milk and cream. Season with salt and pepper. Cook and stir until thick and bubbly, about 3-5 minutes.
Remove from heat and stir in the shredded cheeses until melted.
Pour the cheese sauce over the top of the potatoes. Then spread evenly, allowing the sauce to get into the nooks and crannies around the potatoes.
Sprinkle with extra cheese or breadcrumbs, if desired.
Cover with aluminum foil. Bake at 350° for 45 minutes. Remove the foil and bake another 50 minutes or until potatoes are tender and the top is golden brown and bubbly.
Sprinkle with a little flaky sea salt, if desired. Serve while still warm from the oven.
Make Ahead: Prepare the potatoes about one day ahead and store, unbaked, in the refrigerator. Remove the dish from the refrigerator while you heat up the oven. You may need to add a 15-20 minutes to the baking time.