1. Combine first 9 ingredients in the bowl of a food processor; process until finely chopped.
2. Place potatoes in a large saucepan; cover with water. Bring to a boil; cook 13 minutes. Add onion, and cook for 2 minutes or until potatoes are tender. Drain; cool 10 minutes. Place potato mixture in a large bowl; mash with a potato masher or fork. Stir in chickpea mixture and breadcrumbs; cover and chill 8 hours or overnight.
3. Divide potato mixture into 8 equal portions, shaping each portion into a 1/2-inch-thick patty (about 2/3 cup mixture). Heat 1 tablespoon oil in a large skillet over medium heat. Add 4 patties to pan; cook 5 minutes on each side or until browned and heated through. Repeat procedure with remaining 1 tablespoon oil and 4 patties. Place 1/3 cup spinach and 1 patty in each pita half. Top each serving with 1 onion slice.