Prepare the red potatoes by washing and slicing them thinly to about 1/16-inch thickness. Soak in cold water for at least 30 minutes, then drain and pat dry.

Thinly slice the shallots and set them aside.

Preheat your oven to 375°F (190°C) and grease a 13 x 9-inch baking dish with butter.

In a bowl, mix heavy cream, salt, black pepper, garlic powder, mustard powder, nutmeg, and red pepper flakes until well-blended.

Layer potato slices in the bottom of the greased dish, slightly overlapping. Drizzle with about 1/4 of the cream mixture and sprinkle with green onions and shallots.

Continue layering with more potatoes, cream mixture, and shredded cheeses, pressing down firmly.

For the top layer, add a final set of potato slices, pour remaining cream mixture, and sprinkle with remaining cheeses.

Cover the baking dish with foil and bake for 40-45 minutes. Remove the foil and bake for an additional 20-30 minutes until golden brown.

Let the dish rest for 10 minutes before serving. Garnish with chopped green onions and a sprinkle of black pepper.