Preheat & Prep. Start by preheating the oven to 350 degrees F. Set out a 9x13 casserole dish.
Prep Potatoes. If you haven't already, wash the potatoes and use a mandoline slicer or sharp knife to cut the potatoes into 1/8-inch (3mm) thick slices. (Take your time with this step to get it right. Thicker potatoes will take longer to cook!) Place potatoes in a bowl of cold water if needed, while you prep the sauce.
Heat Garlic. In a medium saucepan, melt the butter over medium heat. Add garlic and cook 1-2 minutes, or until fragrant. Mix in the heavy cream, whole milk, and seasonings and stir to combine and heat through. Create The First Layer. Layer about 1/2 of the thinly sliced potatoes into the casserole dish as evenly as you can. Pour 1/2 of the warm cream mixture over the potatoes in the baking dish and sprinkle with 1 cup of the gruyere cheese.
Create The Second Layer. Layer the second half of the potatoes and top with the second half of the cream mixture, but DO NOT top with the remaining cheese.
Cover Pan Tightly With Foil, so that no steam will escape while cooking.
Bake 40-45 minutes, then uncover.
Top With The Remaining Gruyere Cheese and the Parmesan Cheese.
Bake another 40-45 minutes or until the sauce is bubbling, the top is golden brown, and the potatoes are fork tender (you can easily pierce the potatoes with a thin knife or a fork.) If needed, you can bake the potatoes longer till they're cooked through. If the potatoes begin to brown too quickly, you can loosely tent the pan with the aluminum foil until the potatoes are tender. If desired, you can broil the potatoes 1-2 minutes for a darker top--but watch them closely so they don't burn!
Finish the potatoes by letting them cool about 5-10 minutes before serving. Garnish with fresh parsley or fresh thyme.
Serve & Store. Use a thin metal spatula to serve the layered potatoes. Keep in mind that the cream will be thinnest when it's piping hot, but it will thicken as the potatoes cool a bit. Store leftover potatoes covered or in an airtight container in the fridge 2-3 days.