For the Mint Yogurt, mix yogurt, mint and cumin in small bowl. Cover. Refrigerate until ready to serve.
For the Lamb Skewers, mix lemon juice, oil, rosemary, thyme, garlic powder, sea salt and pepper in small bowl. Reserve 1/2 of the marinade for basting. Place lamb in large resealable plastic bag or glass dish. Add remaining marinade; toss to coat well.
Refrigerate 1 hour or longer for extra flavor. Remove lamb from marinade. Discard any remaining marinade. Alternately thread lamb, tomatoes and onion wedges onto skewers.
Grill over medium-high heat 10 to 12 minutes or until desired doneness, turning occasionally and brushing with reserved marinade. Serve with Mint Yogurt.