Heat the grill to high. Put the potatoes in a pan of cold salted water and bring to the boil. Once boiling, cook for 4-5 mins until nearly tender, then add the asparagus for a final 1 min. Drain.
Meanwhile, heat the oil in an ovenproof frying pan and add the onion. Cook for about 8 mins until softened.
Mix the eggs with half the cheese in a jug and season well. Pour over the onion in the pan, then scatter over the asparagus and potatoes. Top with the remaining cheese and put under the grill for 5 mins or until golden and cooked through. Cut into wedges and serve from the pan with salad.