When you serve this dish, have the toast waiting on warmed plates, and call the diners to the table as soon as you have put the liver in the pan – it takes only minutes to cook and tastes best when eaten sizzling hot.
Using a sharp knife or scissors, remove any gall bladders and green-stained sections that are still attached to the livers. Wash the livers well in cold water. Drain and pat them dry with paper towels. Slice the livers into 5-mm-thick slices.
Heat the oil and butter in a medium frying pan on medium heat until the butter is foaming. Add the onion and cook, stirring occasionally, until the onion is soft. Remove the onion to a plate with a slotted spoon. Cover with foil to keep warm.
Meanwhile, combine the flour, salt, pepper and paprika and dip the livers into the seasoned flour. Add the bacon or ham fat to the frying pan and heat on medium-high heat. Add the livers and fry them for 1–2 minutes on each side, or until they are a light golden colour. Return the onions to the pan and stir until they are well combined with the livers and heated through.
Drizzle the liver and onions with the lemon juice and season to taste with salt and pepper. Spoon quickly over the warm toast and serve immediately while piping hot.