Combine first 4 ingredients. Divide mixture into 4 equal portions, shaping each into a 1/2-inch-thick patty.
Place patties on a grill rack coated with cooking spray; grill 6 minutes on each side or until a thermometer registers 165°. Remove from grill; let stand 5 minutes. Cut each patty in half. Split each pita in half horizontally, cutting to, but not through, opposite side. Spread 1 tablespoon Minted Pea Hummus on bottom half of each pita, and top with half of 1 patty. Add onion, sprouts, and cucumber, if desired.