First wash and dry the liver. It's normally already in pieces so I didn't bother chopping it.
Put the flour into a shallow bowl or plate and season well. So around 1tsp or salt and ½ teaspoon of pepper.
Now coat the liver in the flour.
Heat a tablespoon of butter in a large frying pan on medium heat.
Fry the liver for 3-4 minutes on either side so that its nice and brown.
Set the liver aside.
Keep the left over flour for now, we're going to use it later.
Add another tablespoon of butter to the pan. Fry the onions and bacon until the bacon is cooked and the onions are starting to brown.
Sprinkle the remaining flour to the pan and mix everything together.
Pour in the stock and the ketchup and mix together.
Now thicken the sauce for around 5-10 minutes.
Add in the liver, mix and lid the pan.
Turn the heat right down and cook for 15 minutes.
Add salt and pepper to taste and enjoy with mash or rice.