In a 9-inch cast-iron skillet, heat the olive oil until shimmering. Add the meatballs and cook over moderate heat, turning, until golden but not quite cooked through, 5 to 6 minutes. Transfer to a plate. Add the tomato sauce to the skillet and simmer until slightly thickened, about 2 minutes. Return the meatballs to the skillet and simmer, turning, until just cooked through, about 2 minutes longer. Transfer to a bowl, sprinkle with the parsley and serve.