Preheat the oven to 375°F (190°C) and prepare a baking dish with cooking spray.

In a skillet, melt butter over medium heat. Add sliced onions with a pinch of salt and cook until golden brown (approximately 20 minutes).

In a bowl, whisk together heavy cream, garlic, thyme, salt, and pepper.

Layer half of the potato slices in the baking dish, followed by half of the caramelized onions and half of the cheese. Repeat these layers.

Pour the cream mixture over the layered ingredients until just covered.

Cover with aluminum foil and bake for 45 minutes. Remove foil and bake for an additional 15-20 minutes until bubbly and golden.