Preheat the oven to 400°F (200°C) and grease a large baking dish with unsalted butter.

Peel and thinly slice the Yukon gold potatoes, aiming for even slices about 1/8 inch thick.

In a large pot, melt 4 tablespoons of unsalted butter over medium heat and add minced garlic, stirring for about 1 minute until fragrant.

Sprinkle in the all-purpose flour and whisk to form a roux, cooking for 1-2 minutes until golden.

Gradually pour in the heavy cream while whisking, and cook for about 5 minutes until it thickens slightly.

Stir in the grated Parmesan, fresh thyme, kosher salt, and black pepper until smooth.

Combine the sliced potatoes with the sauce, gently tossing until evenly coated.

Layer the potato slices vertically in the greased baking dish and pour any remaining sauce over the top, adding extra Parmesan.

Cover with aluminum foil and bake for 30 minutes, then remove the foil and bake for an additional 30 minutes until golden.

Allow to rest for about 10 minutes before serving to let the sauce settle.