Heat the vegetable oil in a heavy non-stick frypan over high heat. Cook and stir the garlic, ginger and onion in the hot oil until fragrant, about 1 minute; add the pork mince to the mixture and cook until completely browned, 7 to 10 minutes. Drain any excess liquid from the frypan.
Reduce the heat to medium. Add the carrot and celery to the pork mixture; cook and stir until the carrot and celery begin to soften, 2 to 3 minutes. Stir the soy sauce and spring onions into the mixture; remove the frypan from the heat and add the hoisin sauce and pine nuts. Stir the mixture until evenly mixed. Fold the watermelon into the mixture.
Spoon the pork mixture into the middle of the lettuce leaves; wrap the lettuce around the filling to serve.