Preheat the oven to 375°F (190°C). Peel and thinly slice the russet potatoes.

Melt butter in a medium saucepan over medium heat and sauté diced onions and minced garlic for about 5 minutes.

Whisk in all-purpose flour and cook for 1 minute, then gradually add heavy cream while stirring until thickened for 4-5 minutes.

Lower heat and stir in most of the shredded cheese until melted and smooth.

Grease a baking dish, layer half the potatoes, then half the cheese sauce. Repeat with remaining potatoes and cheese sauce, finishing with reserved cheese on top.

Cover with aluminum foil and bake for 40 minutes. Remove the foil and continue baking for 20-25 minutes until golden-brown.

Let the dish rest for 10 minutes before serving to allow the sauce to thicken.