Preheat oven to 400°F.

Scrub potatoes well then slice into ⅛" thin slices using a mandoline. Slice onion as thin as possible, using a mandoline for uniform slices.

Layer potatoes and onions in little stacks and place upright in a buttered/greased 2.5-3 quart casserole dish.

Melt butter in a saucepan then add garlic and saute until fragrant. Add the flour and cook for 2-3 minutes over medium low heat, stirring constantly.

Add the seasonings and whisk in the half and half. Continue whisking until thick and bubbly. Remove from heat and stir in cheeses until melted.

Spoon cheese sauce over potatoes. Cover with foil (sprayed with cooking spray) and bake for 1 hour.

Remove foil and bake an additional 20-30 minutes or until lightly browned and potatoes are cooked. Cool 15 minutes before serving.