How to Make It
Light a grill. On a work surface, using a sharp knife, cut the tuna into thin strips. Stack the strips and cut them into cubes. Add the minced bacon and chop until the tuna is fine and the bacon is evenly mixed in. Transfer the tuna mixture to a bowl. Stir in 1/4 cup of the lemongrass rub and season lightly with salt and pepper. Using slightly moistened hands, form the tuna mixture into four 4-inch patties. Brush the tuna patties with oil and season with salt and pepper. Grill the burgers over moderately high heat, turning once, until browned on the outside, about 6 minutes.
Meanwhile, combine the mayonnaise with the remaining 1 tablespoon of the lemongrass rub and season with salt and pepper. Set the tuna burgers on the rolls and serve with the aioli and onion slices.