Prepare the Salisbury Steaks: In a large mixing bowl, combine the beef mince, grated onion, minced garlic, breadcrumbs, beaten egg, Worcestershire sauce, Dijon mustard, dried thyme, black pepper, and salt. Use your hands to thoroughly mix the ingredients until just combined – be careful not to overmix, as this can make the patties tough. The mixture should feel well-bound and slightly sticky.

Form and Sear the Patties: Divide the mixture into 6-8 equal portions and shape each into an oval or round patty, about 1.5 cm thick. Heat the olive oil in a large, heavy-bottomed frying pan or skillet over medium-high heat until it shimmers. Carefully place the patties into the hot oil, ensuring they don't touch. Sear them for 3-4 minutes per side until a beautiful golden-brown crust forms and they release easily from the pan, filling your kitchen with a rich, savoury aroma. Don't worry about cooking them through; we're just building flavour here. Remove the seared patties to a plate and set aside.

Start the Mushroom Gravy Base: In the same pan, reduce the heat to medium. Add the butter and let it melt until it sizzles gently. Add the sliced chestnut mushrooms and chopped onion. Sauté, stirring occasionally, for 7-10 minutes until the mushrooms have softened, released their moisture, and taken on a lovely golden hue, and the onions are translucent and fragrant. You'll notice the delightful earthy smell of the mushrooms intensifying.

Build the Gravy's Richness: Add the minced garlic to the pan and cook for another minute until fragrant. Sprinkle the plain flour over the mushrooms and onions, stirring constantly for 2 minutes to cook out the raw flour taste. The mixture will become thick and pasty, clinging to the vegetables. This roux is essential for thickening the gravy.

Simmer the Gravy: Gradually pour in the hot beef stock, whisking continuously to prevent lumps. Stir in the tomato purée, Worcestershire sauce, dried thyme, and black pepper. Bring the gravy to a gentle simmer, stirring often. It should start to visibly thicken and become smooth, glossy, and a rich brown colour.

Combine and Finish: Carefully return the seared Salisbury steak patties to the pan, nestling them into the simmering mushroom gravy. Ensure they are mostly submerged. Reduce the heat to low, cover the pan, and let it gently simmer for 20-25 minutes. The patties will finish cooking through, absorbing the magnificent flavours of the gravy, becoming incredibly tender. The gravy will also thicken further to a lovely, spoonable consistency. Taste and adjust seasoning with salt and pepper as needed. Before serving, sprinkle with fresh chopped parsley, if using. The sight of the glistening gravy and tender steaks is truly inviting.