This is a classic in Sweden, and I think in UK as well(?). This is the way I make it.
The steaks are marinated with grated onion for a few hours.
Lots of sliced onion is slowly sautéed in butter.
When the onion is browned, a little wheat flour and dark sugar is added, then fried for a few minutes more.
Beef stock is added, Worcestershire and HP Sauces to taste (careful!). Salt and pepper and a dash of soy sauce for colour. Transfered to a pot, the gravy is kept warm.
The grated onion is carefully scraped off the steaks (it would burn otherwise). The steaks are seasoned with salt and pepper and fried in butter.
Served with a generous amount of onion gravy, steamed potatoes and pickled cucumber.