STEP 1
Heat a large frying pan over a medium-high heat. Rub the oil all over the beef joint and season well with sea salt. Put the beef, garlic and onion halves in the pan, and brown everything on all sides until evenly coloured. Transfer everything to the slow cooker.
STEP 2
Tip the stock into the pan and bring to the boil, scraping up any browned bits. Whisk in the miso paste and tomato purée, and pour the mixture into the slow cooker.
STEP 3
Tip the peppercorns, bay and rosemary into the slow cooker, cover and cook on high for 1 hr 30 mins-1 hr 40 mins, or 2 hrs 20 mins on low until the beef reaches 60C when tested with a probe thermometer. Once cooked, carefully transfer to a plate, cover and leave to rest for 30-40 mins.
STEP 4
While the beef rests, strain the cooking liquor into a jug, squeezing all the liquid from the onions. Skim off any fat that rises to the surface. Mix 4 tbsp of the liquor with the cornflour, then tip this into the slow cooker with 500ml of the remaining liquor. Cook for 10-15 mins, stirring often until you have a smooth, glossy gravy. Alternatively, cook in a pan over a medium heat for 3-5 mins. Strain into a jug remove any lumps, then season with sea salt to taste. Serve alongside the beef for pouring over.