Preheat oven to 375°F.

Slice potatoes using a mandolin for uniform thickness (about ⅛-inch thick). Try to keep the shape of each potato intact while slicing. It's easiest to slice directly onto a cutting board. Set aside.

Layer the sliced butter in the bottom of a 12-inch cast-iron skillet.

Carefully stack and layer the sliced potatoes over the butter, maintaining their shape as much as possible.

Strip the leaves from 2 sprigs of thyme and sprinkle the leaves over the potatoes. Reserve the remaining thyme for garnish after baking.

Sprinkle the garlic, salt, pepper, 2 cups of the shredded cheese (reserve ½ cup), and chopped bacon evenly over the potatoes.

Pour the heavy cream evenly over the top.

Cover the skillet tightly with aluminum foil and bake for 35 minutes.

After 35 minutes, remove the foil, sprinkle the remaining ½ cup of cheese on top, and return to the oven. Bake for an additional 30–40 minutes, or until the potatoes are tender and the top is golden brown.

Remove from the oven, garnish with the remaining thyme leaves, and serve immediately.