Make the patties: In a large bowl, combine the beef mince, grated onion, minced garlic, breadcrumbs, beaten egg, tomato purée, Worcestershire sauce, dried thyme, salt, and pepper. Use your hands to mix gently until everything is just combined — don't overwork it, or the patties will turn dense and tough. You should see the mixture holding together without being sticky.

Shape and sear: Divide the mixture into 4 equal portions and shape each into an oval patty, about 2cm thick. Heat the oil in a large, heavy-based frying pan over a medium-high heat until it shimmers. Carefully lay the patties in the pan — you should hear a satisfying sizzle. Cook for 4-5 minutes on each side until a deep, rich brown crust forms. The patties will feel firm to the touch when flipped. Transfer them to a plate; they won't be cooked through yet, and that's fine.

Sauté the mushrooms: Reduce the heat to medium. Add the butter to the same pan, scraping up any browned bits from the bottom with a wooden spoon. Add the sliced mushrooms and cook for 5-6 minutes, stirring occasionally. You'll see them release their liquid, which will bubble away, and then they'll start to turn a lovely golden colour and smell nutty and earthy. Once they're soft and browned, they're ready.

Build the gravy: Sprinkle the flour over the mushrooms and stir well to coat every piece. Cook for 1-2 minutes, stirring constantly — the mixture will look like a thick, sandy paste and smell slightly toasty. Slowly pour in the beef stock while whisking vigorously to prevent lumps. Add the Dijon mustard and stir. Bring the gravy to a gentle simmer — you'll see small bubbles breaking across the surface — and let it cook for 2-3 minutes until it thickens enough to coat the back of a spoon.

Finish cooking the patties: Return the patties to the pan, nestling them into the gravy. Spoon some gravy over the tops. Reduce the heat to low, cover the pan with a lid, and let everything simmer gently for 10 minutes. The gravy will deepen in colour and become glossy. The patties should be cooked through but still tender — cut one open to check it's no longer pink in the centre.

Rest and serve: Remove the pan from the heat and let it sit, covered, for 5 minutes. This allows the patties to relax and absorb some of that gorgeous gravy. Sprinkle with fresh parsley just before serving. The aroma at this point is absolutely irresistible — rich, savoury, and deeply comforting.