Finely chop the onion.
Mix ground beef and ground pork together.
Add breadcrumbs, milk, egg, salt, and pepper, and mix well.
Knead by hand and shape into round, flat patties.
Add oil to a pan and cook the Hamburg steaks over medium-low heat.
Cook both sides until golden brown.
Pour demi-glace sauce over the steaks and simmer together.
Plate and serve.