Step 1: Preheat oven to 375°F (190°C). Grease a 9×13 inch baking dish with softened butter.
Step 2: In a medium saucepan, combine heavy cream, milk, minced garlic, salt, pepper, and nutmeg. Heat over medium until just simmering, then remove from heat.
Step 3: Arrange half of the potato slices in the prepared baking dish, overlapping slightly. Pour half of the cream mixture over the potatoes.
Step 4: Sprinkle half of the Gruyère and Parmesan cheeses evenly over the potatoes.
Step 5: Repeat layers with remaining potatoes, cream mixture, and cheeses.
Step 6: Cover dish tightly with aluminum foil and bake for 45 minutes.
Step 7: Remove foil and bake uncovered for another 20-25 minutes until potatoes are tender and top is golden brown.
Step 8: Let rest for 10 minutes before serving. Garnish with fresh thyme if desired.