Preheat oven to 400 degrees F
Place skillet back on stove over low heat. Melt butter, then cover bottom of pan with 1/3 of potatoes. Season with salt and pepper. Cover potatoes with 1/2 of onion mixture. Cover with 1/2 of remaining potatoes and season with salt and pepper. Spread with the rest of onion mixture. Cover with remaining potatoes, and season with more salt and pepper.
Remove from oven and use a spatula to carefully transfer potatoes to a serving platter.
Sprinkle with chopped parsley just before serving.