Marinade: Combine maple syrup, horseradish, mustard, ancho chile powder, 1/4 teaspoon salt and 1/8 teaspoon black pepper in small bowl; set aside.
Relish: Combine avocados, onions, jalapeno peppers, 3 tablespoons cilantro, the lime juice, olive oil, 1/4 teaspoon salt and 1/8 teaspoon black pepper in medium bowl; set aside.
Quesadillas: Place the Marinade in large resealable plastic bag. Add turkey; seal bag. Turn bag over to evenly coat turkey with marinade. Refrigerate 30 minutes to 1 hour to marinate, turning bag occasionally.
Spray cold grate of outdoor grill with grilling spray. Prepare grill for medium heat. Remove turkey from marinade; discard bag and marinade. Grill turkey 14 to 18 minutes, or until tenderloins are no longer pink in centers (170°F), turning occasionally. Place on cutting board; let stand 10 minutes. Cut into thin slices.
Place 8 of the tortillas on a clean work surface; top evenly with cheese, onion slices and turkey. Then stack 4 of the prepared tortillas (layers) on the other 4 prepared tortillas (layers).Cover with the remaining 4 tortillas. Brush tops with half of the canola oil. Place, oil-sides down, on grill grate. Cook 2 to 3 minutes, or until golden brown on undersides. Brush remaining tops with the remaining canola oil and carefully turn quesadillas over. Grill until cheese is melted and undersides are golden brown. Remove from grill. Cut each quesadilla into 4 wedges. Serve with the Avocado-Red Onion Relish.