Flavorful meatball sandwiches topped with a cool, tangy sauce

1 pound ground lamb(or beef)
1/4 cup minced fresh mint
1/8 cup coarsely grated onion
2 garlic cloves, minced
1.5 tablespoons paprika
1/2 tablespoon ground cumin
3/4 teaspoons coarse kosher salt
.5 teaspoon freshly ground black pepper
1/4 teaspoon cayenne pepper
6 5- to 7-inch-diameter soft round Middle Eastern flatbreads (preferably with no pocket)
olive oil
1 large onion, sliced
roasted red peppers, garnish (optional)

1/2 cup plain low-fat yogurt
1 tablespoons tahini (sesame seed paste)*
1/2 tablespoon fresh lemon juice
1/4 teaspoon salt

Line large rimmed baking sheet with plastic wrap. Gently mix lamb and next 8 ingredients in large bowl. Using moistened hands and scant 2 tablespoonfuls for each, roll meat mixture into 1 1/2-inch meatballs. Arrange on sheet.
Preheat oven to 300°F. Place large sheet of foil on work surface. Heat large nonstick or cast-iron skillet over medium-high heat. Add flatbreads to skillet 1 at a time; cook until browned, about 2 minutes per side. Enclose in foil; place in oven to keep warm.
Heat 1 tablespoon oil in same skillet over medium-high heat. Add onions; sprinkle lightly with salt and pepper. Sauté until golden brown, about 8 minutes. Transfer onions to 1 side of large rimmed baking sheet; place in oven to keep warm.
Add 1 tablespoon oil to same skillet; heat over medium-high heat. Sauté half of meatballs until just cooked through, 7 to 8 minutes. Transfer to baking sheet in oven. Repeat with remaining meatballs, adding oil to skillet if dry. Reserve skillet.

Stir all ingredients in medium bowl to blend. Cover and chill.