Begin by slicing your yellow onions into thin, even rounds. Heat a large skillet over medium heat and add the onions, cooking them until they turn golden brown and develop a sweet aroma—this should take about 15-20 minutes. Stir occasionally to prevent burning.

Using a mandoline slicer or a sharp knife, thinly slice your Yukon Gold potatoes, aiming for uniform thickness for even cooking. Place sliced potatoes in a bowl of cold water to prevent browning. After about 10 minutes, drain and pat them dry.

In a spacious mixing bowl, combine heavy cream with salt and pepper to taste. Whisk until well blended, then fold in the grated Gruyère and Parmesan cheese.

Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish. Start layering by placing a single layer of sliced potatoes at the bottom, followed by caramelized onions and a drizzle of the creamy cheese sauce. Continue this process until all are used, with the final layer covered with remaining sauce and cheese.

Cover the baking dish tightly with aluminum foil and bake for about 1 hour. Remove foil to allow the top to crisp up, and bake for an additional 10-20 minutes until golden and bubbly.

Once baked, let it cool for about 10 minutes before serving. Sprinkle with fresh thyme or chives to enhance flavor and presentation.