Combine the cheeses in a large bowl. Transfer 1/3 of the cheese blend to a small bowl and set aside. To the cheeses in the large bowl, add the cream, garlic powder, thyme, salt pepper, and potato slices (you might have to do this in batches, if your bowl isn’t quite big enough). Toss with your hands (enjoy this!) until each slice is coated with the cream mixture. Taste the cream – if it tastes well-seasoned, then you’re good to go. If not, add a little more salt. 

Grease a 2-quart casserole dish with butter or non-stick spray. Organize a big handful of the cream-coated potatoes into a tidy stack, and lay them in the casserole dish with their edges aligned vertically. Continue placing potatoes in the dish, working around the perimeter and into the center until they’re all in (accordion style), or the dish is totally full (your potatoes should be tightly packed; see note). Pour some of the excess cream/cheese mixture evenly over the potatoes until the mixture comes about 1/3 to halfway up the sides of the potatoes – no more! It will spill over if it gets too full. 

Place the dish on a sheet pan covered in foil. Cover the casserole tightly with foil and place in the oven. Bake for 40 minutes. Remove the foil and continue baking until the top is a light golden brown, about 30 more minutes. Carefully sprinkle with that remaining cheese waiting in the wings, and return to the oven. Bake until deeply golden brown and crisp on top, about 30 minutes. Let it rest out of the oven for at least 10 minutes before serving.