Yogurt Dressing: In small bowl, combine yogurt, green onion, jalapeno (if using), lime juice and cumin.
Cover and refrigerate.
Place sausages, peppers and onion on greased grill over medium-high heat; close lid and grill, turning sausage and onion, until no longer pink inside, about 10 minutes.
Cut sausage into rounds and vegetables into strips.
Wrap tortillas in foil; place on grill and warm through, about 3 minutes.
To serve, divide sausage, peppers, onion, spinach and feta cheese among tortillas; top with yogurt dressing.