Combine mustard and tomato sauce.
Season the beef mince with salt, pepper and cumin and form into a patty a little larger than your burger bun.
Heat a frying pan with oil. Add the beef patty into the pan and place the sliced onions on top of the patty. Season the onions with salt. Cook until the bottom is browned and crisp. Flip patty over, so that the onions are now underneath.
Layer sliced pickles and cheddar cheese on top of the patty and cover with a lid. Cook until cheese has melted.
Remove patty from frying pan. Keeping the fat in the pan and place brioche buns into the pan, cut side down and toast until lightly browned. Remove.
Add a little more oil to the pan if needed and when hot crack in an egg and fry until crispy around the edges. Season with salt.
Build your burger: Divide the ketchup-mustard sauce between the buns, then starting with the bottom bun, layer with letuce tomatoes (sprinkle the tomatoes with salt), followed by the cheesy beef patty, beetroot, fried egg and finish with brioche bun top.