Preheat an oven to 180 degrees C. 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 chilli powder in a small bowl. Mix well. Fill each toasted bun with tuna salad, grated lettuce, mozzarella cheese and one pineapple ring.
Place sandwiches on a baking tray, and heat until the cheese melts, approximately 10 to 12 minutes.