Preheat the oven to 180°C/350°F/gas 4. Season the oxtail with sea salt and a generous pinch of black pepper, then brown all over in a casserole pan on a medium heat for 10 minutes, turning occasionally while you peel and slice the onions. Remove the oxtail to a plate, add 1 tablespoon of oil and the onions to the pan, and cook for 5 minutes, stirring regularly. Return the oxtail to the pan, stir in the flour to coat, then add 4 tablespoons of Worcestershire sauce and the stock. Bring to the boil, then cover and place in the oven for 3 hours to 3 hours 30 minutes, or until the meat is super tender. Strain the gravy into a separate pan and put aside, then tip the meat and onions back into the pan. Once cool enough to handle, carefully pick through the meat, discarding the bones and any wobbly bits, then gently mix back with the onions and a splash each of gravy and Worcestershire sauce, being careful not to break up the meat too much. Pick, roughly chop and stir through the parsley leaves (reserving the stalks). For the burger sauce, finely chop the gherkins, tomato, little gem and reserved parsley stalks, then transfer to a bowl, stir in the yoghurt and season to perfection. Shape the minced meat into four equal-sized patties, being quite rough so they’re not too compact, then season the outsides with salt and pepper. I like to cook two burgers at a time to achieve perfection, so get two pans on the go – a large non-stick pan on a high heat for the burgers and another on a medium heat for the buns. Place two patties in the large pan, pressing them down slightly with a fish slice, then cook for 8 minutes in total, turning every minute and brushing with a little English mustard after the first turn. Put the gravy and oxtail pans on a low heat to warm through. When the burgers are almost cooked, halve and toast two burger buns in the second frying pan. Finely slice the cheese and place on top of each burger, add a splash of water to the pan, then cover with a lid or a metal bowl and cook for a further 30 seconds, or until the cheese has melted. To assemble the burgers, spoon a quarter of the burger sauce on to each toasted bun base, sit a burger on top, spoon over 1 tablespoon of the pulled oxtail (save the rest for another day) and place the bun lid on top. Serve the gravy in a bowl on the side for dunking. Repeat with the remaining two burgers.