Prepare the patties: In a large bowl, combine the beef mince, grated onion, minced garlic, breadcrumbs, beaten egg, Worcestershire sauce, salt, pepper, and thyme. Mix gently with your hands until just combined — you should feel the mixture become cohesive but not pasty. Overworking will make the patties tough. Divide into four equal portions and shape each into an oval patty about 2cm thick. You'll notice the patties feel slightly soft but hold their shape.

First sear for colour and crust: Heat 1 tablespoon of oil in a large, heavy-based frying pan over medium-high heat until it shimmers and a drop of water sizzles immediately. Carefully place the patties in the pan — you should hear a satisfying hiss. Cook for 4 minutes without moving them. The bottoms will turn a deep mahogany brown and develop a crisp crust you can see peeking at the edges. Flip and cook for another 3 minutes. The second side will be slightly lighter in colour but still beautifully browned. Transfer the patties to a plate — they will look quite brown on the outside and feel firm to the touch, but they are not fully cooked through yet.

Caramelise the onions: Reduce the heat to medium and add the remaining 1 tablespoon of oil to the pan. Add the sliced onions and a pinch of salt. Stir to coat the onions in the pan juices and any browned bits on the bottom. Cook for 8–10 minutes, stirring every minute or so. The onions will first soften and release their moisture, then turn translucent, and finally develop golden edges. The smell will become sweet and rich. When the onions are a deep golden colour with some brown spots, they are ready — this usually takes about 10 minutes.

Sauté the mushrooms: Add the sliced mushrooms to the pan with the onions. Increase the heat slightly to medium-high. Cook for 5–6 minutes, stirring occasionally. The mushrooms will first release their liquid — you'll see the pan become wet and steamy. As the liquid evaporates, the mushrooms will begin to brown and shrink. They are done when they have a golden-brown colour, a slightly chewy texture when pressed with a spatula, and the pan is nearly dry again.

Build the gravy: Sprinkle the flour over the mushrooms and onions. Stir constantly for 1 minute — the flour will absorb the remaining fat and juices, forming a thick paste that smells nutty and toasted. You'll see the mixture clump together and become slightly darker in colour. Slowly pour in the hot beef stock while whisking vigorously to prevent lumps. The mixture will immediately thicken and become glossy. Bring the gravy to a gentle simmer — you'll see small bubbles breaking the surface all over, not just at the edges. Let it cook for 2 minutes, stirring occasionally, until the gravy has thickened enough to coat the back of a spoon.

Finish the patties in the gravy: Return the seared patties to the pan, nestling them into the gravy. Spoon some of the gravy over the tops of each patty. Reduce the heat to low, cover the pan, and let everything simmer gently for 10 minutes. The patties will finish cooking in the steam and gravy, becoming tender and juicy. You'll know they are done when the gravy has reduced slightly and thickened further, and the patties feel firm but spring back slightly when pressed. The kitchen will smell deeply savoury and comforting.

Rest and serve: Remove the pan from the heat. Stir in the butter until it melts into the gravy, giving it a silky sheen. Let the dish rest, uncovered, for 5 minutes. During this time, the juices redistribute within the patties. Taste the gravy and adjust seasoning if needed — it should be rich, savoury, and well-balanced. Serve the patties with a generous ladle of mushroom onion gravy over the top, garnished with fresh parsley.