Spray an 18cm frying pan with oil and stir-fry mushrooms in batches until browned.
Drain mushrooms through a sieve to remove excess liquid.
Return mushrooms to pan and stir in garlic, chives, and pepper for one minute.
Preheat the grill to its highest setting.
Pour beaten eggs over mushrooms and cook on low heat for five minutes until almost set.
Transfer the pan to the grill for 3-4 minutes until the top is fully set.
Toss lettuce, tomatoes, and cucumber in a bowl to make the salad.
Loosen the frittata with a knife, cut into wedges, and serve with the salad.