Add the pork, apple, fennel, oregano, onion, garlic, and salt to a bowl. Mix well with your hands, until everything's fully combined.
Shape into 4 patties and place onto a plate, then leave to rest in the fridge for 15 minutes.
Spray a frying pan with low-calorie cooking spray and set on a medium heat. Add the burgers, then fry for 5 minutes on each side — until cooked through.
Spread a little mayonnaise onto each roll and top with the burgers, lettuce and tomato. Serve alone or with an accompaniment of your choice.