Finely chop the onion.
Mix the ground beef and ground pork together.
Add onion, breadcrumbs, egg, milk, salt, and pepper, then knead into a dough.
Shape the mixture into round, flat patties.
Add oil to a pan and cook the Hamburg steaks.
Once the surface is cooked, flip them and cook through to the center.
Pour demi-glace sauce over the Hamburg steaks to finish.