1 garlic clove, smashed
butter, room temperature
3 lbs russet potato, peeled and sliced 1/8 inch thick
2 1/2 cups heavy cream
course salt and pepper
ground nutmeg

Try this

add browned bacon
top with Gruyere cheese after removing foil
add fresh thyme leaves to cream mixture

I honestly love a potato gratin without onion. Often the onion overwhelms the great flavors versus enhancing them.

Preheat oven to 375F, rub a 2-quart baking dish with garlic, then lightly butter. Layer potatoes in dish, overlapping slightly.

In a medium saucepan, combine cream, 1 1/2 tsp salt, 1/4 tsp pepper, and a pinch of nutmeg; bring to simmer. Pour enough cream mixture into dish to cover potatoes; cover with foil. Bake until potatoes are tender when pierced with a pairing knife, 40-45 minutes. Remove foil and continue to bake until top of gratin is golden, 35-40 minutes. Let rest for 15 minutes before serving.