Cook onion in butter over medium heat for 15 minutes or until onions are very soft and lightly browned, stirring frequently. Add zucchini and cook for 5 minutes more. (May be prepared up to 2 days ahead and refrigerated until ready to use.) Stir in chicken, peppers and seasoning.
Lay tortillas on a flat surface and place equal amounts of cheese, onion mixture and walnuts over half of each; fold over to enclose. Cook in a large skillet coated with nonstick cooking spray for 5 minutes on each side or until tortilla is crisp and cheese is melted.
Cut into wedges and serve with warm marinara sauce, if desired. Makes 8 appetizer servings.