Rinse sweet potato and place in a shallow microwave-safe dish no more than two layers deep. Cover and microwave on High/100% for 4-6 minutes or until almost tender. Drain and set aside.
Heat the oil in a 26cm round (5cm deep) ovenproof, non-stick frying pan over a medium heat. Add onion and cook, stirring for 5-8 minutes, or until soft.
Add the sweet potato to the hot pan, cook for 5 minutes, stirring occasionally until potatoes are light golden. Spread the mixture evenly over the base of the pan. Top with half the spinach. Sprinkle with cheese. Reduce heat to medium-low.
Whisk the eggs in a large bowl until well combined. Add parsley and season with pepper. Pour the egg mixture over potato and spinach, gently shake pan to allow the egg to run between the potato pieces. Cook for 8-10 minutes until edges are firm but the top still a little soft. Remove from the heat.
Preheat a grill on medium. Place the frittata (still in the frying pan) under the grill and cook for 5 to 7 minutes, or until top is firm and light golden. Loosen the edges and slide onto a serving plate or board. Cut into wedges. Serve with the remaining spinach.