1 pound boneless lamb shoulder, trimmed and cut into 1-inch pieces
2 tablespoons olive oil, divided
3/4 cup finely chopped onion
1/4 cup finely chopped shallots
2 tablespoons minced garlic, divided
3/4 teaspoon ground cumin
3/4 teaspoon ground coriander
1/4 teaspoon ground cardamom
1/4 teaspoon ground mustard
1/4 teaspoon ground turmeric
1/8 teaspoon ground red pepper
Dash of grated whole nutmeg
2 tablespoons finely chopped fresh mint, divided
3/4 teaspoon kosher salt, divided
2 red bell peppers
1/2 cup 2% low-fat Greek-style plain yogurt
1 tablespoon fresh lemon juice
1/4 teaspoon freshly ground black pepper, divided
Cooking spray
4 (1 1/2-ounce) hamburger buns, toasted
2 cups torn radicchio