Make the seasoned flour: Combine the flour, paprika, mustard powder, salt, and black pepper, and spread out on a shallow plate.
Dredge the liver: Dredge the liver slices in the seasoned flour until completely covered on both sides. Set aside.
Sauté the onions: Heat a large cast iron skillet on medium high heat. Add a teaspoon of the bacon fat (or butter). Sauté the onions until translucent, a couple of minutes. Remove onions from pan with a slotted spoon. Set aside onto a serving dish.
Fry the liver slices: Add the remaining 2 teaspoons of bacon fat (or butter) to the skillet. Working in batches, if necessary, add the liver slices. Fry until browned on both sides, no more than 3 to 4 minutes on each side. Serve hot with sautéed onions (and ketchup on the side!).
