Prepare the Hamburger Steaks: In a large mixing bowl, combine the ground beef, egg, breadcrumbs, garlic powder, onion powder, salt, and black pepper. Using your hands, gently but thoroughly mix the ingredients until just combined. Overmixing can lead to tough steaks. Divide the mixture into 4-6 equal portions and form them into oval or round patties, about 1.5 cm thick. They should hold their shape well.

Sear the Steaks: Heat 1 tablespoon of vegetable oil in a large, heavy-bottomed frying pan or skillet over medium-high heat until shimmering. Carefully place the hamburger steaks into the hot pan, ensuring they don't overcrowd the pan (you may need to do this in batches). Sear for 4-5 minutes per side until a beautiful, deep brown crust has formed and the steaks are cooked through with no pink remaining in the middle. The aroma of seared beef will fill your kitchen. Remove the cooked steaks from the pan and set them aside on a plate, loosely covered with foil to keep warm.

Sauté the Onions: Add the remaining 1 tablespoon of oil to the same pan, reducing the heat to medium. Add the thinly sliced onions. Sauté, stirring occasionally, for 8-10 minutes until they soften significantly and turn a lovely translucent golden-brown colour. You'll hear a gentle sizzle, and their sweet, pungent smell will become more mellow.

Create the Roux: Sprinkle the plain flour over the softened onions. Stir continuously for 1-2 minutes, allowing the flour to cook out. The flour will absorb the oil and onion juices, forming a thick paste or 'roux'. This step is crucial for thickening your gravy and removing any raw flour taste; you'll notice a slightly nutty scent.

Build the Gravy: Gradually whisk in the hot beef stock, a little at a time, ensuring each addition is fully incorporated before adding more. This prevents lumps. As you whisk, the gravy will begin to thicken and become smooth. Stir in the soy sauce and Dijon mustard. Bring the gravy to a gentle simmer, stirring occasionally, and let it cook for 5-7 minutes, until it has visibly thickened to a rich, glossy consistency that coats the back of a spoon. Taste and adjust seasoning as needed.

Combine and Serve: Return the cooked hamburger steaks to the pan, nestling them into the simmering onion gravy. Allow them to heat through for another 2-3 minutes, absorbing some of that delicious gravy. The steaks will look deeply coated and inviting. Garnish with fresh chopped parsley, if using, and serve immediately.