Preheat the oven to 400 degrees F. Place the potatoes in a pot of salted water. Bring the potatoes up to a boil and blanch for 2 minutes. Remove the pan from the heat and drain. In a large oven-proof saute pan, heat the olive oil. When the oil is hot, add the onions. Season with salt and pepper. Lower the heat to medium and saute the onions until caramelized, about 8 to 10 minutes. Add the garlic and heat until fragrant about 2 minutes more. Turn the onion mixture into 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. Cover the first layer of potatoes with 1/2 of the onions. Cover the onions with 1/3 of the potatoes. 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. Slice the lyonnaise into 12 slices. Garnish with parsley.