Take the liver and toss in the seasoned flour until it is coated all over; put to one side.
Add the oil to a large frying pan and bring up to medium heat. Add the onions and fry gently until they start to caramelise slightly, add garlic, and fry for about another 5 minutes. Remove everything from the pan and put to one side.
Bring the pan back up to a high heat till just smoking and add the liver, brown the liver quickly on both sides - a minute for each side should be enough - and when done add the onions, garlic. Mix together then set to one side.
Over a medium heat add a little flour to the pan (you may need to add a splash more oil) and stir until a roux, or thick paste, is formed. Add the water and using a whisk mix well.
Lower heat and add stock cubes (crumbled), Worcestershire sauce, tomato puree, tomato ketchup, salt and pepper and mix well.
Turn heat up and add gravy granules whilst stirring until it starts bubbling then lower the heat and add the liver and onions and mix well.
Cover and simmer for 5 to 10 minutes then check liver is cooked correctly (if there is a pink liquid rising out of it and it's soft when you put a sharp knife in it, then its done. It should have a texture like firm pate.)