Preheat your oven to 375°F (190°C). In a medium saucepan, melt 3 tablespoons of butter over medium heat, then add 3 tablespoons of flour, whisking continuously for 1 minute until smooth and bubbly. Gradually whisk in 2 cups of heavy cream and cook, stirring constantly, until it thickens and bubbles, about 5-7 minutes.

Stir in 1 teaspoon of salt, 1/2 teaspoon of black pepper, and 1/2 teaspoon of garlic powder. Reduce heat to low and add 1 1/2 cups of shredded cheddar cheese and 1 cup of shredded Gruyère cheese, stirring until melted and smooth.

Grease a 9x13 inch baking dish with butter or cooking spray. Arrange the thinly sliced potatoes in overlapping rows at the bottom, then evenly distribute the yellow onions over the potatoes. Pour the cheese sauce evenly over the arranged potatoes and onions, ensuring full coverage. Sprinkle the remaining 1/2 cup of shredded Gruyère cheese over the top.

Cover the dish with aluminum foil and bake for 40 minutes. Remove the foil and bake for an additional 20-25 minutes, or until the top is golden brown and the potatoes are tender. Once baked, let it cool for a few minutes, then garnish with freshly chopped parsley or chives before serving. Enjoy!