Form the patties. In a large bowl, combine the beef mince, breadcrumbs, beaten egg, grated onion, minced garlic, tomato purée, dried thyme, salt, and pepper. Use your hands to mix gently until just combined — overmixing will make the patties dense and tough. You should feel a soft, cohesive mixture that holds together without being sticky. Divide into 6 equal portions and shape them into oval patties, about 2cm thick.

Sear the patties. Heat the vegetable oil in a large frying pan over a medium-high heat until it shimmers and a drop of water sizzles on contact. Carefully place the patties in the pan — you should hear a satisfying sizzle. Cook for 2-3 minutes per side until a deep, golden-brown crust forms. The patties will release easily from the pan when they are properly seared. If they stick, give them another 30 seconds. Transfer the seared patties to the slow cooker insert, arranging them in a single layer.

Deglaze the pan. Pour about 100ml of the beef stock into the hot frying pan. Use a wooden spoon to scrape up all the browned bits from the bottom — these are packed with flavour. Let it bubble for about 30 seconds until it reduces slightly and smells deeply savoury.

Make the gravy base. Pour the deglazed liquid into the slow cooker, around the patties. Add the remaining 400ml of beef stock and the Worcestershire sauce. Stir gently to combine, being careful not to break the patties.

Slow cook. Cover the slow cooker with the lid and cook on LOW for 6 hours, or on HIGH for 3 hours. After this time, the patties should be fork-tender and the liquid will have darkened to a rich, mahogany brown. The smell at this point is deeply savoury and comforting.

Thicken the gravy. Carefully remove the patties to a plate and cover to keep warm. Whisk the cornflour slurry into the hot liquid in the slow cooker. Turn the slow cooker to HIGH, replace the lid, and cook for a further 15-20 minutes, until the gravy has thickened to a silky, spoon-coating consistency. If using, stir in the butter for a glossy finish. Return the patties to the gravy to warm through for 2-3 minutes before serving.