Heat 1 tablespoon of the oil in a large frying pan and fry the bacon rashers until lightly browned on both sides. Drain on paper towels and keep warm. Add the remaining oil to the pan and fry the onions until lightly browned, 10–15 minutes; push them to the side of the pan.
Meanwhile, mix the flour with the salt and pepper on a plate. Dip the liver, a slice at a time, in the seasoned flour until evenly coated, shaking off the excess. Add the butter to the pan and heat over medium heat until sizzling.
Add the liver, sprinkle with half the sage leaves and fry for 2 minutes, or until lightly browned. Turn each piece and cook the other side, sprinkling with the remaining sage. Remove the liver from the pan and set aside.
Stir any remaining flour into the pan, pour in the Madeira and stir, scraping any browned residue off the bottom of the pan. Add the stock and bring to the boil, stirring occasionally. Return the liver to the pan, reduce the heat and simmer for 4–5 minutes, or until the liver is tender but still slightly pink.
Return the bacon to the pan and serve immediately with rice or mashed potatoes and a steamed green vegetable, if desired.