Melt remaining 2 tablespoons butter in pan over medium heat. Add cooked rutabagas and pack down with spatula. Cook, without moving, until underside of rutabagas are brown, 7 minutes. Turn the rutabagas over with a spatula, pack down again, and continue to cook until well browned and crisp on the opposite side, another 7 minutes. Reduce heat to medium-low and continue cooking, stirring rutabagas every few minutes, until crusty and golden on all sides, 9 to 12 minutes. Stir in onion, garlic powder, salt, and pepper to taste.