Step 1

Preheat the oven to 350 degrees F. Butter a large casserole dish.

Step 2

Heat the butter in a large skillet over medium heat. Add the onion and bacon, then cook until the bacon is crisp and the onions have caramelized, about 10 minutes.

Step 3

Pour the cream and half-and-half into a saucepan and set over low heat to warm until the mixture is no longer cold. Whisk in the flour, pepper and salt until totally combined. Set aside.

Step 4

Combine the two grated cheeses in a bowl. Set aside.

Step 5

Using a mandoline, slice the potatoes into 1/8-inch-thick slices. Layer a third of the potato slices in the prepared casserole dish. Sprinkle on a third of the bacon and onion mixture, then a third of the cheese; pour in a third of the cream mixture. Repeat this twice more, ending with a sprinkling of cheese and the rest of the cream mixture. Cover the dish with foil and bake for 40 minutes. Remove the foil and bake until bubbly and hot, about 20 minutes more.

Step 6

Sprinkle on the green onions and serve.