Preheat an oven to 350 degrees. Toast hamburger buns and spread cut sides with butter.
Drain half of the oil from the tuna. Combine the tuna and remaining oil, lemon juice, salt, pepper, and chili powder in a small bowl. Mix well. Fill each toasted bun with tuna salad, shredded lettuce, mozzarella cheese, and one pineapple ring.
Place sandwiches on a baking sheet, and heat until the cheese melts, approximately 10 to 12 minutes.