Prepare the sauce by mixing the ketchup, soy sauce, Worcestershire sauce, and sugar until fully combined and set aside. You should have about 1/2 cup of tonkatsu sauce.
Mince the onion.
Heat a frying pan on medium heat with olive oil. Saute the chopped onions until they become translucent. Set aside once it's cooked.
In a large bowl, mix together ground beef, pork, sauteed chopped onions, milk, panko breadcrumbs, 1 egg, ketchup, salt, nutmeg, and black pepper.
You have to knead this meat mixture until it gets sticky and has the texture of hamburger patties.
Now divide the meat mixture into 6 equal-sized balls.
After that, shape each ball into a meat patty. You must toss the patty between your hands about 5 or 6 times to remove air bubbles. Each ground meat patty should have the shape of a minced cutlet.
Set up three bowls for the egg wash, flour, and panko breadcrumbs.
In the first bowl, whisk together 2 eggs. In the second bowl, add 1/2 cup of all-purpose flour. In the third bowl, add 1.5 cups of panko breadcrumbs.
First, coat each patty in flour, then dip it in the beaten egg wash, and finally coat it in panko breadcrumbs.
Make sure each patty is evenly coated with the breadcrumbs.
Place the panko-coated patties on a plate or cutting board, and let them rest.
Heat up the vegetable oil to about 340 degrees F in a deep fryer or a large saucepan.
Carefully place the meat patties in the hot oil, and deep fry for 3 minutes on each side.
Remove the patties and let the excess oil drain on paper towels and a cooling rack.