In a large bowl gently mix beef, egg, breadcrumbs, Worcestershire sauce, onion powder, garlic powder, salt and pepper.
Divide into 3 or 4 portions and shape into large patties. They will shrink when cooked so can be shaped fairly large.
Heat oil in large frying pan over medium-high heat.
Add patties and cook until nicely browned, about 4 minutes.
Flip patties over and add onion. Continue cooking until patties are browned on both sides. Leave onion in the pan and transfer patties to a plate. Cover with foil while onions continue cooking.
Stir onions occasionally, scraping up brown bits on the bottom of the pan. Cook until softened, about 10 minutes total.
Add beef stock and sherry.
While stock is heating, whisk cornstarch and water together in a small bowl.
Pour cornstarch mixture into pan and stir immediately until sauce thickens, 1-2 minutes.
Return beef patties to the pan, reduce heat to low and cook for 10-15 minutes until the meat is cooked through.
Taste and adjust for seasoning.