In a medium mixing bowl, whisk the marinade ingredients together and place in a ziplock bag with the lamb leg steaks. Place in the fridge to marinate for at least 1 hour preferably overnight.
Add 2 tablespoons of olive oil to a non-stick frying pan or cast-iron skillet. Once the oil is hot, it will shimmer. Take the lamb out of the marinade and place it in the hot pan. Cook for about 4 minutes on each side until the lamb is golden and slightly charred around the edges. Leave the lamb on a plate and cover with foil to rest.
While the lamb is resting, make the dressing. Place the Greek yogurt in a medium mixing bowl and stir in the shredded cucumber. Mix in the olive oil, lemon juice, and herbs. Season with salt and pepper. Stir in the garlic. Add some water to loosen the dressing until you have your desired consistency.
Assemble the salad by placing the romaine lettuce in the bottom of a salad bowl and season with some salt and drizzle with the tzatziki dressing. Layer the cucumber slices, cherry tomatoes, olives, and feta over the lettuce. Slice the lamb, and place it on top of the salad. Drizzle with the dressing and some extra mint leaves. Serve with pita chips.