Preheat your oven to 350°F (175°C).

Wash and slice the Yukon Gold potatoes into 1/8-inch rounds and set aside in a bowl of cold water.

In a medium saucepan, melt the butter over medium heat and sauté the onion and minced garlic for 1-2 minutes until fragrant.

Sprinkle in flour, stirring for about 1 minute to form a roux. Gradually add milk and chicken broth while whisking until thickened, about 3-5 minutes.

Stir in thyme and salt to taste, bringing the sauce to a gentle boil.

Turn off the heat and mix in the cheddar, mozzarella, and parmesan cheese until melted and smooth.

In a greased 9x13 casserole dish, layer potato slices and pour cheese sauce over, repeating two more times.

Cover with foil and bake for 30 minutes; then uncover and bake for an additional 35-45 minutes until golden brown and fork-tender.

Let the dish rest for about 10 minutes before serving.