Prepare the panade: In a large bowl, combine the breadcrumbs, 60ml of milk, Worcestershire sauce, and ketchup. Let it sit for 5 minutes until the breadcrumbs have absorbed the liquid and formed a thick, soft paste. This is the secret to tender, not dense, patties.

Form the patties: Add the beef mince, grated onion, garlic, beaten egg, salt, and pepper to the panade. Use your hands to gently mix everything until just combined – over-mixing will make the meat tough. The mixture should feel tacky but not sticky. Divide into 6 equal portions and shape each into an oval patty, about 2cm thick. You should hear a slight squelch as you form them.

Brown the patties: Heat the vegetable oil in a large, heavy-based skillet over a medium-high heat until it shimmers. Carefully place the patties in the pan, ensuring they aren't touching. Sear for 3-4 minutes per side until a deep, dark brown crust forms. You'll hear a satisfying sizzle, and the patties should release easily from the pan when ready. Transfer them to a plate; they will not be cooked through yet.

Cook the mushrooms: In the same pan, add the sliced mushrooms. Let them cook undisturbed for 2 minutes until they begin to brown and release their moisture. You'll see the pan juices darken and the mushrooms will start to look glossy and shrunken. Stir and cook for another 2-3 minutes until they are soft and deeply golden.

Make the gravy base: Sprinkle the flour over the mushrooms and stir constantly for 1 minute. This roux will turn a pale golden colour and smell slightly nutty. Slowly pour in the warm beef stock while whisking vigorously to prevent lumps. The mixture will thicken immediately. Add the Dijon mustard and thyme sprigs.

Simmer the patties: Return the patties to the pan, nestling them into the gravy. The gravy should come about halfway up the sides of the patties. Bring to a gentle simmer (you should see small bubbles breaking the surface, not a rolling boil). Reduce the heat to low, cover the pan partially with a lid, and let it cook for 12-15 minutes. The gravy will darken and thicken further, and the patties will be cooked through and tender.

Rest and serve: Remove the pan from the heat. Let the patties rest in the gravy for 5 minutes. This resting time allows the juices to redistribute, ensuring every bite is moist. The gravy should have a silky, coating consistency.