Classic home cooking for a large group of people. You may find it easier to cook this dish in separate batches.
Put 400g/14oz of the flour into a large bowl and season with salt and freshly ground black pepper.
Heat half the oil in a large frying pan over a high heat.
Coat the liver in the seasoned flour, shake off any excess, then fry each slice in the hot fat for 1–2 minutes on each side until lightly browned, but not completely cooked through.
Transfer the slices to a plate and repeat with the remaining liver.
Reduce the heat, add rest of the oil and the sliced onion and cook for a minute or so, stirring occasionally.
Add the bacon to the pan and cook for 8–10 minutes or until the onion is a pale golden-brown and the bacon is beginning to crisp.
Sprinkle the rest of the flour over the onion and bacon and stir for a minute or so. Add the stock, bring to a simmer and cook over a medium heat until the gravy is thickened. Add the tomato ketchup and season with salt and freshly ground black pepper.
Return the liver to the pan with the onion gravy for just 1–2 minutes to warm it through and finish cooking.
