1-Begin by greasing a 9-inch square baking dish. Arrange the 2 ½ pounds of sliced Yukon gold potatoes and ½ medium yellow onion in slanted rows with space between the slices for even cooking.

2-In a saucepan, melt the 3 tablespoons of salted butter over medium heat. Whisk in the ¼ cup of all-purpose flour to form a paste, then gradually add the 1 ½ cups of whole milk or unsweetened almond milk, stirring until the mixture thickens.

3-Stir in the 8 ounces of shredded sharp Cheddar cheese, ¾ teaspoon kosher salt, ½ teaspoon garlic powder, and freshly ground black pepper until smooth and creamy.

4-Pour the cheese sauce over the arranged potatoes and onions in the baking dish. Cover the dish with foil and bake at 375°F for 45 minutes.

5-Uncover the dish, sprinkle the top with 2 ounces of shredded Gruyere cheese and 2 ounces of grated Parmesan cheese, then bake for an additional 30 to 45 minutes until golden and tender.

6-For extra crispness, broil for 1 to 2 minutes. Let the dish rest for 10 minutes, then garnish with chopped fresh chives or parsley before serving warm.