Preheat your oven to 375°F (190°C) and grease a 9x13-inch baking dish.

Melt 4 tablespoons of unsalted butter in a saucepan over medium heat. Add chopped onion and sauté until soft.

Stir in minced garlic and cook for an additional 30 seconds.

Add 1/4 cup of all-purpose flour to the saucepan and whisk continuously for 1-2 minutes to make a roux.

Gradually whisk in 2 cups of whole milk and 1 cup of heavy cream while bringing to a gentle boil.

After 3-5 minutes, the sauce should thicken. Remove from heat and stir in 1 cup of cheddar cheese and 1/2 cup Parmesan cheese.

Layer half of the sliced potatoes in the baking dish and pour half of the cheese sauce over them.

Repeat with the remaining potatoes and cheese sauce.

Sprinkle the remaining cheddar and Parmesan cheese on top.

Cover with foil and bake for 30 minutes, then uncover and bake for another 25-30 minutes until golden and fork-tender.

Let the dish rest for 10 minutes before serving and garnish with parsley if desired.