Cut 2 lemons into 4 wedges and set aside. Juice the remaining lemons. Place the lemon juice, oil, garlic and salt and pepper into a small jug. Whisk well to combine.
Place mushrooms into a large bowl and lamb into a medium shallow dish. Pour 1/2 the marinade over mushrooms and toss well to combine. Pour remaining marinade over lamb and turn lamb to coat in marinade. Set mushrooms and lamb aside to marinate for 15 minutes.
Preheat a greased barbecue plate and grill on medium-high heat.
Place lamb onto barbecue plate and cook for 2 -3 minutes (for medium rare) on each side or until cooked to your liking. Remove lamb to a plate, cover with foil and set aside to rest.
Meanwhile, place mushrooms onto barbecue plate and cook for 3-4 minutes or until just tender. Remove to a plate. Cook lemon wedges.
Slice lamb across the grain into thick slices. Arrange lamb, mushrooms and salad leaves onto serving plates. Serve with barbecued lemon wedges.