Hooray for cheezy comfort food! The key to this recipe is to slice waxy potatoes ethereally thin (no more than 1/8", ideally more like 1/16"). I accomplished this with the slicing side of my grater. I tried using the food processor, but it didn't slice them thin enough. Even if you can't get them that thin, it will still taste good, but the texture won't be as nice.
Preheat the oven to 350°. Grease an 7"x11" baking dish.
1 quart very thinly sliced peeled, waxy potatoes (such as red potatoes or Yukon gold)
1 can Cream of Onion Soup
1.5 c. milk
3/4 c. grated Swiss cheese
3/4 c. grated cheddar cheese (mix cheeses together to form 1.5 c)
2 Tbsp. butter
3 Tbsp. flour
1 tsp. ground mustard
1 tsp. paprika
1 tsp. black pepper
1 tsp. seasoned salt
1/2 tsp. garlic powder
1/4 tsp. cayenne
Make a roux: heat the butter in a small pan until it starts to shimmer, then add the flour. It will bubble up; stir frequently until it is a slightly browned paste, taking care not to let it scorch.
Whisk the canned soup, milk, and spices over medium high heat, until it starts to simmer at the edges. Whisk in the roux and continue to simmer until the sauce thickens. Stir in 1 c. of the cheeses, reserving 1/2 c. to sprinkle on top. Adjust the seasonings as desired.
Lay out 1 c. of the potato slices on the bottom of the pan so that they overlap slightly. Ladle a thin layer of cheese sauce over the potatoes, and then repeat, until you have 4 layers of potatoes and a layer of cheese sauce on top (only 3 layers of potatoes if your slices are thick). Sprinkle the remaining cheese on top, and sprinkle with some paprika for color.
Bake for 1 hour, or until the top is browned and bubbly and the potatoes are tender. The potatoes on the bottom of the pan should have formed a lovely browned crust (make sure they don't burn).
You can always spice this up a bit by adding chives, bacon, more spices, whatever. If you don't want to make a roux then you could thicken the sauce with corn starch, but it won't have as much flavor and it won't hold as well in the fridge (if indeed you have any leftovers).