I adapted this from a Bon Appetit magazine. I love ground lamb and have tried countless recipes to find the best lamb burger. The original recipe came close, then I added a few elements that I liked from various other recipes and came up with this.
Combine lamb, 1/2 cup onions, 1 teaspoon salt, pumpkin pie spice, pine nuts, and 1/4 teaspoon cayenne pepper in medium bowl. Mix gently. Using moistened hands, form lamb mixture into 4 equal patties, each about 1/2 inch thick.
Combine yogurt, cumin, remaining 1/2 cup onions, 1/4 teaspoon salt, and 1/4 teaspoon cayenne pepper in small bowl. Stir to blend. Let stand 15 minutes.
Prepare barbecue (medium-high heat). Grill burgers until cooked to desired doneness, about 4 minutes per side for medium.
Carefully open pita breads to make pockets; insert burger in each. Spoon in yogurt mixture and tomatoes, dividing equally, and serve.