Preheat the oven to 400 degrees F (200 degrees C).

Peel potatoes and cut them into 1/2-inch slices. Place in a pot and cover with water; bring to a boil. Boil for 2 minutes, then drain and set aside.

Heat a large oven-proof skillet over medium-high heat. Pour in olive oil, then add onions. Sauté until lightly caramelized, 8 to 10 minutes. Stir in garlic and sauté until onions are deep brown and garlic is soft. Transfer mixture to a bowl.

Place the skillet back on the stove over low heat. Melt butter in the skillet, then cover the bottom with 1/3 of the potatoes. Season with salt and pepper. Cover potatoes with 1/2 of the onion mixture. Cover with 1/2 of the remaining potatoes; season with salt and pepper. Spread remaining onion mixture on top. Cover with remaining potatoes; season with salt and pepper.

Bake in the preheated oven until potatoes are tender and browned on top, 10 to 12 minutes.

Use a spatula to transfer potatoes to a serving platter. Sprinkle with chopped parsley just before serving.