This is an adaptation of a recipe in The Food Lab cookbook. The idea is to combine the concepts of hasselback potatoes (thinly sliced potatoes placed with the slices vertical and with some fat and flavorings between the slices) and scalloped potatoes. By placing the potato slices vertically in the pan, there are lots of protruding bits to get browned as the casserole roasts. This was served to general acclaim for Christmas Eve dinner along with a roasted pork loin.
A key to success is to properly size the pan for the amount of potatoes and other ingredients so that the potato slices will fill the pan when placed vertically. I found that 3 lbs. of potatoes filled a small lasagna pan, and that was enough to feed 6 people.
3 oz. medium textured cheese (e.g. cheddar), coarsely grated
2 oz. grating cheese (parmesan, romano, etc.), finely grated
1-1/2 cups cream
1 clove garlic, finely chopped
1 Tbs. fresh thyme, copped
Kosher salt and freshly ground black pepper
3 lbs. potatoes, peeled and thinly sliced
2 Tbs. butter
Preheat the oven to 400 degrees. Grease well the baking dish with the butter.
Mix together the cheese. Reserve one third of the cheese into a separate small bowl to sprinkle on top at the end, and put the rest of the cheese into a large bowl. Add the cream, garlic, and thyme to the bowl. Season well with salt and pepper. Add the sliced potatoes and mix with the cream mixture with your hands until every surface of the potato slices is coated, which will take several minutes. Place the potato slices vertically in the baking dish and then pour the remaining cream mixture from the bowl into the baking dish.
Cover the dish with aluminum foil and roast for 30 minutes. Remove the foil and roast for another 30 minutes. Sprinkle on top the reserved cheese and roast for a final 30 minutes. When done the potato slices will be fully cooked, the cream mixture will have reduced, and the top of the potatoes will be nicely browned.