Preheat the oven to 350°F (175°C). Grease a 9×13 inch baking dish with butter or nonstick spray.

Melt the butter in a skillet over medium heat. Add the chopped onion and minced garlic. Cook for 4 to 5 minutes until the onion is soft and translucent. Remove from heat.

In a large bowl, whisk the heavy cream with a generous pinch of salt and a few grinds of black pepper.

Arrange half of the potato slices in an even layer in the prepared baking dish.

Pour half of the cream mixture over the potatoes. Sprinkle half of the grated cheddar on top. Spread half of the sautéed onion and garlic over the cheese.

Add the remaining potato slices in a second layer. Pour the rest of the cream over the potatoes. Finish with the remaining cheddar and the rest of the onion mixture. Press lightly so layers settle.

Cover the dish tightly with aluminum foil. Bake for about 45 minutes.

Remove the foil and bake uncovered for another 15 to 20 minutes until the top is golden and a fork slides into the potatoes easily.

Let the casserole rest for 10 minutes before serving so the sauce sets. Garnish with chopped parsley if desired.