1Rinse potatoes and place in a shallow microwave-safe dish no more than two layers deep. Cover and microwave on High/100% for 4 minutes or until potatoes are almost tender. Drain and set aside.
2Heat half the oil in a heavy-based large 5cm deep 20cm (base), non-stick frying pan over medium heat. Add the mushrooms and cook 5-8 minutes, stirring often until almost tender. Remove to a plate lined with paper towel. Add remaining oil and potatoes to the hot pan, cook, shaking the pan for 8 minutes until potatoes are light golden. Return the mushrooms to the pan and spread over the base of the pan. Scatter over the ham.
3Whisk the eggs in a bowl until well combined. Add parsley and season with pepper. Pour the egg over the mushroom mixture, gently shake the pan to allow the egg to run between the potato and mushrooms. Sprinkle with parmesan. Cook for 10 minutes or until the base and edges are firm but the top still a little soft. Remove from the heat. Spoon large dollops of ricotta over the frittata.
4Preheat a grill on medium heat. Place the frittata (still in the frying pan) under the grill and cook for 5 minutes or until top is firm and light golden. Season with salt and pepper. Loosen the edges, cut into wedges and serve wedge of lemon.