Mince the onion.
In a large bowl, mix in the onion, ground meat, oatmeal, Worcestershire sauce, milk, egg, salt, and pepper.
Knead well with hands until the meat mixture becomes pale and sticky.
Divide into 5 - 6 patties.
Toss from one hand to another to release the air in the patties.
Prepare 3 separate bowls for the flour, beaten eggs, and panko.
Dredge and coat each patty in flour first, then the egg, and last the panko. You can lightly press the patties into the panko to get a nice coating.
Put around 2 -3 patties in air fryer at a time. Coat each side evenly with the olive oil spray.
Air fry at 360F-370F on each side for about 4 min. Turn the heat up to 390F for an additional 2 min to get a nice golden brown crust.
Serve with thin sliced cabbage and tonkatsu sauce.