Preheat the oven to 400°F and spray a 2.5–3 quart casserole dish with non-stick cooking spray.

Layer 3 pounds russet potatoes peeled and sliced into ?-inch thick slices and 1 thinly sliced white onion into the casserole dish, arranging them in slightly slanted rows and overlapping the potatoes to allow room for the sauce to flow between layers.

In a medium saucepan over medium-high heat, melt ¼ cup butter.

Stir in ¼ cup all-purpose flour until combined.

Slowly whisk in 2 cups whole milk, ½ teaspoon garlic powder, 1 teaspoon salt, and ½ teaspoon pepper. Cook for 2–3 minutes, whisking continuously, until the sauce thickens.

Remove the saucepan from the heat and immediately stir in 1 cup shredded sharp cheddar cheese and ½ cup shredded Gruyère cheese until melted and smooth.

Pour the cheese sauce evenly over the potatoes and onions, making sure all of the potatoes are coated.

Cover the casserole dish tightly with foil and bake for 60 minutes.

Remove the foil and continue baking for 20–30 minutes, until the top is browned and the potatoes are fork tender.

Let the potatoes rest for 20–30 minutes before serving.

Top with chopped fresh parsley and enjoy!