Form the Patties: In a large mixing bowl, gently combine the beef mince, finely diced onion, minced garlic, beaten egg, breadcrumbs, dried parsley, salt, and pepper. Use your hands, but be careful not to overwork the mixture. Divide into four equal portions and shape them into oval patties, about 2cm thick.

Sear the Beef Patties: Heat the olive oil in a large, heavy-based frying pan or skillet over a medium-high heat. Carefully place the patties in the pan, ensuring not to overcrowd it (work in batches if necessary). Cook for 3-4 minutes on each side until a deep brown crust has formed. The patties won't be cooked through at this stage. Remove them from the pan and set them aside on a plate.

Sauté the Onions and Mushrooms: Reduce the heat to medium and add the butter to the same pan. Once melted, add the sliced onions. Cook for 8-10 minutes, stirring occasionally, until they are soft, sweet, and nicely caramelised. Add the sliced mushrooms and cook for another 5-7 minutes until they have released their liquid and started to brown.

Make the Roux: Sprinkle the plain flour over the onions and mushrooms. Stir continuously and cook for one minute. This step cooks out the raw flour taste and is essential for a thick, glossy gravy.

Build the Gravy: Gradually pour in the warm beef stock, whisking constantly to prevent any lumps from forming. Once all the stock is incorporated and the gravy is smooth, stir in the Worcestershire sauce and Dijon mustard.

Simmer and Thicken: Bring the gravy to a gentle simmer. It will begin to thicken as it heats. Let it bubble gently for 2-3 minutes. Have a taste and add a little more salt or pepper if you think it needs it.

Finish the Dish: Return the seared hamburger steaks (and any juices from the plate) to the pan, nestling them down into the gravy. Reduce the heat to low, cover the pan with a lid, and let it all simmer gently for 10-15 minutes. What works best for me is spooning some of the gravy over the patties halfway through. This ensures they are cooked through and have absorbed all that wonderful flavour. Serve immediately.