In a small bowl, combine tuna, mayo, minced onion, celery seed, relish and black pepper. Set aside. Preheat skillet. Spread margarine on sides of bread and lightly brown all 4 slices in skillet. Remove browned bread to a platter and place 2 slices each of american cheese on 2 pieces of bread. Top each with half of the tuna mixture. Cover remaining 2 slices cheese each and 1 slice browned bread each. Return to skillet on medium heat and cook til cheese is melted and tuna is warm in center - about 4 minutes each side. Return to plate, slide lengthwise and serve with sweet potato fries (homemade or frozen).