HEAT the oil in a frying pan over medium heat and cook the sweet potato for 10 minutes, stirring occasionally. ADD the spinach and cook, stirring occasionally, for another 2 minutes. WHISK the eggs in a bowl with the salt, pepper and half the Parmesan cheese. POUR the egg mixture into the frying pan and shake the pan gently to distribute the ingredients. COOK the frittata over a medium heat for about 6 minutes. WHILE the frittata cooks, preheat the oven broiler (griller). SPRINKLE the top of the frittata evenly with the reserved Parmesan cheese and cook under the broiler for 2 minutes. CUT the frittata into wedges and serve warm or at room temperature.