Step 1) Combine fennel, rosemary, orange peel, orange juice and oil in a small bowl. Salt and pepper to taste. Reserve half of salad in a separate bowl.
Step 2) Place pork tenderloins in a shallow baking pan and cover with remaining salad as a marinade; turn to coat. Cover and marinate in the refrigerator for 1 hour, turning occasionally.
Step 3) Preheat grill. Remove pork from marinade; discard any remaining marinade. Cook pork until deeply browned on all sides; 10 to 15 minutes per side, or until cooked to desired doneness.
Step 4) Transfer meat to a cutting board, cover and let rest for 5 to 10 minutes.
To serve, slice across the grain into 1/2-inch thick medallions. Serve with reserved salad and garnish with orange slices and fresh fennel leaves if desired.