In a bowl, mix the ground beef, grated onion, breadcrumbs, egg, ketchup, Dijon mustard, Worcestershire sauce, salt, and pepper until just combined.

Divide the mixture into 4 equal parts and shape them into oval patties about 3/4-inch thick.

Heat olive oil in a large skillet over medium-high heat. Add the patties and cook about 3-4 minutes per side until browned. Remove patties from the skillet and set aside.

In the same skillet, add the sliced onion and minced garlic. Cook for 2 minutes until soft. Then add the mushrooms and cook for about 3 minutes until they start to brown.

Lower the heat to medium and add the butter. Once melted, stir in the flour and cook for 1-2 minutes. Slowly whisk in the beef broth until the mix is smooth.

Let it simmer for about 5 minutes until the gravy thickens. If it thickens too much, add another splash of broth to thin it. Adjust salt and pepper if needed.

Return the patties and any juices to the skillet. Spoon the gravy over the patties and let them heat through for another 5 minutes. Serve warm, ideally over mashed potatoes. Garnish with parsley.