Preheat the oven to 350°F (175°C).

Melt the butter in a skillet over medium heat. Add the chopped onion and minced garlic. Cook, stirring occasionally, until the onion is softened, about 4–5 minutes. Remove from heat.

In a medium bowl, whisk the heavy cream with a pinch of salt and a few grinds of black pepper. Taste and adjust lightly.

Grease a baking dish with butter or nonstick spray. Arrange half of the thin potato slices in an even layer.

Pour half of the cream mixture over the potatoes. Sprinkle half of the grated cheddar across the top. Evenly distribute half of the onion and garlic mixture.

Repeat with the remaining potato slices, rest of the cream, remaining cheese, and remaining onion-garlic mix. Press down gently to compact the layers slightly.

Cover the dish tightly with foil and bake for 45 minutes.

Remove the foil and continue baking for another 15–20 minutes, until the top turns golden and the potatoes are tender when pierced with a fork.

Let the dish rest for 10 minutes to thicken. Garnish with chopped fresh parsley if desired, slice, and serve.