Cut turkey tenderloin into 8 (about 1/2-inch thick) slices.
Place all marinade ingredients into large resealable plastic food bag; add turkey pieces. Tightly seal bag.Turn several times to coat turkey well. Place in 13x9-inch pan. Refrigerate, turning occasionally, at least 30 minutes.
Combine all tzatziki sauce ingredients in medium bowl; mix well. Refrigerate sauce until serving time.
Heat gas grill on medium-high or charcoal grill until coals are ash white.
Remove turkey from marinade; discard marinade. Place turkey slices onto grill. Grill, turning once, 5-7 minutes or until internal temperature reaches 165ºF and is no longer pink. Top each turkey slice with 2 quarters cheese during last minute of cooking.
Cut 1-inch slice off one side of each pita pocket to create small opening. Tuck cut piece of pita inside pocket. Spoon about 1/4 cup tzatziki sauce into each pocket, spreading sauce to coat bread. Place spinach leaves, tomatoes slices and turkey slices into each pocket.
*Substitute boneless skinless chicken breasts.
**Substitute (5-inch) white pita pockets. If you cannot find mini pita pockets, use full-sized pita pockets and cut each in half.