Preheat your oven to 350°F (175°C).

In a large skillet, melt butter over medium heat. Add chopped onions and minced garlic, sauté for 5 minutes until soft.

Wash and slice the potatoes thinly, about 1/8 inch thick. Place them in a bowl of cold water to prevent browning.

In a greased gratin dish, layer a single layer of potatoes, followed by sautéed onions, garlic, pancetta, and porcini mushrooms. Repeat until all ingredients are used, ending with a layer of potatoes.

Pour heavy cream over the layered ingredients, ensuring even coverage. Sprinkle Gruyère cheese on top.

Cover the gratin dish tightly with aluminum foil and bake for 1 hour.

After 1 hour, remove the foil and bake for an additional 15-20 minutes until the top is golden brown.

Let the gratin rest for at least 10 minutes before serving.