Melt the butter over medium heat in a large skillet. Add the potatoes and onion and sauté until tender, about 5 minutes. Season with the salt, pepper, and parsley. Use a spatula to press the potatoes into pan. Cook until a crust forms, about 5 minutes. Using the spatula, break apart the potatoes, press down again, then let stand to form another crust. Cover skillet, reduce heat to medium-low, and cook for 10 minutes more.