Place finely chopped bell peppers in center of a clean dish towel. Gather ends of the towel together, twist tightly, and squeeze over sink to drain as much liquid as possible. Note: Squeezing peppers tightly is important, otherwise the mixture will be too wet to adhere to the skewers.
In a bowl, add bell peppers, 1 pound lamb, 2 minced garlic cloves, 1½ teaspoon salt, 1 teaspoon cumin, 1 teaspoon pul biber, and ½ teaspoon sumac. Using your hands knead until thoroughly combined and mixture starts to get sticky, about 5 minutes. Cover and refrigerate for 30 minutes.
Divide mixture into 8 equal portions. Using lightly moistened hands, shape each portion into 5-inch-long cylinder about 1½ inches in diameter.
Thread each portion onto a 12-inch metal skewer, reshaping as needed and using fingers to flatten meat slightly on two sides and create horizontal indentations. Note: Skewers can be refrigerated for up to 24 hours.
Heat a large shallow grill pan or griddle over medium-high heat for 5 minutes. Add 1 tablespoon oil to pan and heat until just smoking.
Lay the skewers flat side down and cook until well browned and meat thermometer reads 160 degrees, 2 to 3 minutes a side. Note: If using pitas, 2 minutes before removing kebabs from pan, arrange pitas on top of skewers to warm through.
Serve each lavash bread or pita with 2 kebabs per person. Serve with onions.