Place the Tuna steaks in a sealable plastic bag, Whisk the soy sauce, lime juice, canola oil, green onions, Dijon mustard, lime zest, garlic and pepper in a bowl and pour over the steaks. Seal tightly and turn to coat. Marinate in the refrigerator for 3 hours, turning occasionally. Preheat the grill or boiler. Grill over hot coals or broil on a rack in a broiler pan for 3 to 4 minutes per side or to the desired degree of doneness.