In a shallow bowl put the oil, lemon zest and juice, cumin (if used) and thyme leaves, season and gently mix. Add the lamb, coat in the marinade on both sides, cover and set aside for 20 minutes, or if time allows 1-2 hours in the fridge.
Meanwhile, cook the potatoes in a large pan of boiling water for 15-20 minutes, drain and transfer to a large bowl.
Heat a non-stick frying pan or preheat a grill to moderate and cook the steaks for 6-8 minutes on each side. Transfer the lamb to a warm plate to rest for 3-4 minutes.
Meanwhile, add the remaining salad ingredients to the potatoes. In a small clean jar or bowl, combine the dressing ingredients together and pour over the salad.
Spoon the salad onto serving plates and serve with the lamb.