Mix the first 6 ingredients; fold in tuna. Shape into 4 patties.
In a large cast-iron or other heavy skillet, heat butter over medium heat. Cook patties until lightly browned, 4-5 minutes on each side. Serve on buns. If desired, top with lettuce and tomato.
Health Tip: Lighten up this burger by using reduced-fat mayonnaise and brushing the patties with 1 Tbsp. butter. Then use your broiler instead of pan-frying. You’ll save 75 calories and 10 grams of fat per serving.