Preheat the oven to 400° while you prep the potatoes. Rinse and scrub your potatoes to remove any surface dirt then cut ¼” thin slices using a mandolin. Exercise caution when using a mandolin because they can be extremely dangerous. If you don't have a mandolin, a knife will work as well, just make the slices as thin as possible.
Next, add the heavy cream to a medium saucepan and add thyme sprigs, garlic, shallot, salt & pepper. Heat the cream gently over medium low heat, for about 5-7 minutes, stirring occasionally to help infuse the flavors into the cream.
While the cream heats, layer the potatoes into a 13” by 9” baking dish. Stack the potatoes together and place them in the dish with the thin side facing up. Continue stacking and layering the potatoes until the entire dish is filled. Make sure the potatoes are layered together closely so everything bakes evenly.
Once the dish is filled with potatoes, carefully pour the cream over the top, covering the potatoes. You want the liquid to fill about ¾ of the way up the dish.
Cover tightly with foil and bake in preheated oven for 40 minutes. I recommend placing the dish on a baking sheet, just in case anything boils over while cooking.
After the 40 minutes, remove the foil and add the cheese to the top. Continue baking for another 20 minutes until cheese is melted and bubbling. If you want the cheese to be more browned, broil it for a few minutes until it reaches your desired doneness.
Once the potatoes are finished baking, remove from the oven and let sit for 15 minutes before serving.