Spray a small, flame-proof frying pan with oil and place over a high heat. (The base of the pan shouldn’t be wider than about 18cm/7in.) Stir-fry the mushrooms in three batches for 2-3 minutes, or until softened and lightly browned. Tip the cooked mushrooms into a sieve over a bowl to catch any juices – you don’t want the mushrooms to become soggy.
Return all the mushrooms to the pan and stir in the garlic and chives, and a pinch of ground black pepper. Cook for a further minute, then reduce the heat to low.
Preheat the grill to its hottest setting. Pour the eggs over the mushrooms. Cook for five minutes, or until almost set.
Place the pan under the grill for 3-4 minutes, or until set.
Combine the salad ingredients in a bowl.
Remove from the grill and loosen the sides of the frittata with a round-bladed knife.Turn out onto a board and cut into wedges. Serve hot or cold with the salad.