Prepare lamb: Sprinkle lamb with 1 tsp. salt. In a bowl, whisk together oil, vinegar, remaining 1 tsp. salt, the curry powder, garlic, and ginger. Add lamb and toss to coat. Chill, covered, at least 4 hours or overnight.
Prepare yogurt sauce: In a bowl, combine yogurt, garlic, lemon juice, and salt. Chill until used (up to 1 day).
About 30 minutes before cooking, soak six 10-in. bamboo skewers in water and let meat stand at room temperature. Heat a grill to medium-high (350° to 450°), or set a grill pan on the stove over medium-high heat.
Remove lamb from marinade (discard marinade) and thread onto skewers. Grill (covered if on an outdoor grill), turning occasionally, 7 to 8 minutes total for medium-rare.
Serve with yogurt sauce, naan bread, cilantro sprigs, and mint leaves.
* If boned lamb shoulder isn’t available, buy 3 lbs. bone-in shoulder chops. Find Madras curry powder at well-stocked grocery stores and at oaktownspiceshop.com.