2 Cups whole-milk yogurt, divided
For the Tabil Spice Blend (combine):
1 Tbsp + 1 tsp Coriander Seed
2 tsp Caraway Seed
1/2 tsp Garlic Powder
1/4 tsp Cayenne Pepper
1/4 tsp Curry Powder
5 large garlic cloves, minced, divided
1 tsp Kosher Salt plus more for seasoning
1 tsp Ground Black pepper plus more for seasoning
1/4 Cup Extra-Virgin Olive Oil
2 Tbsp fresh lemon juice
1 4–5 pound butterflied leg of lamb, opened like a book
4 lemons, halved
Mix 1 Cup yogurt, 1 teaspoon Tabil Spice Blend, and 1 minced garlic clove in a small bowl. Season to taste with salt and pepper. Cover and chill.
Whisk remaining 1 cup yogurt, remaining Tabil Spice Blend, 4 minced garlic cloves, 1 tsp. salt, 1 teaspoon pepper, oil, and lemon juice in a large bowl. Add lamb to yogurt-spice mixture; turn to coat. Cover and chill overnight, turning occasionally.
Prepare grill to medium-high heat. Season lamb with salt and pepper. Grill until meat is cooked to desired doneness, 10–15 minutes per side for medium-rare. Let rest 10 minutes.
Meanwhile, grill lemon halves, cut side down, until charred, about 5 minutes. Thinly slice lamb against the grain and arrange on a platter; garnish with lemons. Serve with reserved yogurt sauce.