Preheat oven to 400°F.
Combine cheeses and cooked pancetta in a large bowl. Transfer 1 handful of the cheese mixture to a separate bowl and set aside. Add cream, garlic, and thyme to remaining cheese mixture and season with salt and pepper. Add potato slices and toss to combine, making sure the cream mixture coats every potato slice.
Spray a medium casserole dish non-stick cooking spray. Take a handful of potatoes, and stack them on top of each other and then lay them in the casserole dish like a hasselback potato. Continue stacking potatoes in the casserole dish until it's full of potatoes and everything is tightly packed. Pour the leftover cream mixture evenly over potatoes until the mixture comes half-way up the sides of the potatoes, if you have even more leftover liquid after that, just discard.
Cover the casserole dish with aluminum foil and transfer to the oven. Bake for 30 minutes, remove from oven, remove foil and continue baking until top is pale golden brown, about 25-30 minutes more. Once golden, remove from oven, sprinkle with remaining cheese, and return to oven. Bake until deep golden brown and crisp on top, about 30 minutes longer. Remove from oven, let rest for a few minutes, and slice and serve.