Heat the vegetable oil in a large pan or wok (a high sided pan is best to reduce oil spattering). Coat the lamb in the cornflour, and when the oil is hot, add to the pan (you can check it's hot enough by putting one piece of lamb in the pan. If it sizzles, it's hot enough). Cook on a high heat for about 5 or 6 minutes, turning every couple of minutes until the lamb is crispy and brown.
Whilst the lamb is cooking, make the dressing by mixing all of the dressing ingredients together.
Arrange the salad leaves on four plates or one large serving platter.
When the lamb is ready, remove from the pan with a slotted spoon and place on some kitchen roll to soak up any excess oil.
Top the salad leaves with the crispy lamb, then add the sun blush tomatoes. Sprinkle on the red onion slices and feta cheese before serving with the balsamic dressing.