Add the bacon to a large pot, then turn the heat to medium. Fry until crisp with the fat rendered, then remove and place to one side, leaving the excess fat in the pot. Dice just before serving.
Add the onion and celery to the leftover fat (top up with olive oil if you need to) and fry until they start to soften and turn golden. Add the garlic and fry for 30 seconds or so, then add the beef (again, add a dash of oil if you need to). Raise the heat slightly and break it up with your wooden spoon. Try to properly brown the beef to develop flavour (without burning the veg).
Add the salt, pepper and mustard powder, then crush the Oxo/Bouillon cube and stir to coat the beef. Fry for another minute or so, then reduce the heat back down to medium.
Stir in the flour, then gradually add all but 1/2 cup/120ml of the stock, stirring as you go so it blends with the flour without creating lumps. Stir in the milk, then stir in the Worcestershire sauce. Stir in the diced potato, then bring to a simmer. Gently simmer with the lid off until the potato is cooked through and the soup thickens (around 15 mins or so). Make sure the potato is properly cooked through so it soaks in the flavour.
Stir in 1 tsp pickle juice, then turn off the heat and stir through the cheese until it blends into the soup. If at any point you need to thin out the soup, just use the leftover stock to do so to reach your desired consistency.
Check for seasoning and adjust with salt and pepper. Make sure the soup is properly seasoned to bring out the flavour. Serve individual portions with extra cheese, bacon, pickles and sesame seeds, then tuck in and enjoy!