1. For the marinade, whisk the vinegar, tomato paste, oil, lemon zest and juice together in a large flat dish. Stir in the onion, garlic, oregano, bay and black pepper, then add the diced lamb and stir well to coat. Cover and chill for at least 2 hours, but ideally 6 to 8 hours.
  2. When ready to grill, skewer the lamb tightly onto the bamboo skewers and grill over a preheated grill on medium high heat for 8 to 10 minutes for medium doneness. Serve with tztatziki.
  3. For the tzatziki, toss the cucumber with a sprinkle of salt and let it sit 10 minutes. Squeeze the cucumber to draw out excess liquid and stir the cucumber into the yogurt along with the mint, dill and garlic. Keep chilled until ready to serve.