Preheat the oven to 400.
Place the potatoes in a pot of salted water. Bring the potatoes up to a boil and cook for 2 minutes. Remove the pan from the heat, drain and cool. Heat the olive oil in a large ovenproof skillet over medium-high heat. Add the onions and season with salt and pepper. Cook, stirring, until the onions are caramelized, 8 to 10 minutes. Add the garlic, and cook until fragrant, about 1 minute. Transfer the mixture to a bowl. Place the pan back on the stove and melt the stick of butter. When the butter has melted, cover the bottom of the pan with 1/3 of the potatoes.
Season with salt and pepper. Cover the first layer of potatoes with 1/2 of the onion mixture. Cover the onions with 1/3 of the potatoes. Season with salt and pepper. Repeat the layering until all of the potatoes and onions are used. Place the pan in the oven and cook for 10 to 12 minutes or until the potatoes are golden brown. Remove the pan from the oven.
Using a spatula, gently lift the potatoes out of the pan and place on a platter. Garnish with parsley.