Heat gas or charcoal grill. In small bowl, mix butter, 1 tablespoon chopped green onion and the orange peel; set aside.
In another small bowl, mix oil, celery salt and pepper. Brush oil mixture on both sides of tuna steaks and over whole green onions.
Place tuna on grill over medium heat. Cover grill; cook 8 to 10 minutes, turning once, until tuna flakes easily with fork and is slightly pink in center. Add onions to grill during last 5 minutes of grilling; turn occasionally. Cover tuna and let stand 5 minutes before serving. Serve tuna and onions with dollops of orange butter.