Start by placing the russet potatoes in a pot of salted water. Bring to a boil and cook until fork-tender, about 15-20 minutes. Drain them and let them sit for a moment.

In a skillet, melt 2 tablespoons of butter over medium heat. Add the sliced yellow onions and cook, stirring occasionally, until they transform into a beautiful caramelized color, around 15-20 minutes.

In a mixing bowl, combine the drained potatoes with heavy cream, the remaining 2 tablespoons of butter, salt, pepper, and half of your beautifully caramelized onions. Mash together until creamy yet chunky.

Gently fold in the Gruyère cheese and fresh thyme leaves into the potato mixture. Transfer this mix to a greased baking dish. Top generously with the remaining caramelized onions and extra cheese.

Preheat your oven to 375°F. Bake the dish for 25-30 minutes until it's golden brown and the cheese is bubbly.