Preheat the oven to low to keep the bacon warm later. Halve, peel and slice the onion. Finely chop the large sage leaves.
Heat 1 tablespoon oil in a frying pan, add the onion and fry over moderate heat for 4–5 minutes, until lightly browned.
Meanwhile, rinse the liver and pat dry with paper towels. Put the flour onto a plate, season well with pepper, then coat each slice of liver with the flour.
Tip the leftover flour and the chopped sage into the onions and cook, stirring, for 1 minute. Add the stock and bring to the boil, stirring. Reduce the heat; leave to simmer.
Heat the remaining olive oil in a frying pan and fry the bacon for 1–2 minutes on each side, then remove from the frying pan and keep warm in the oven. Add the liver to the frying pan and fry over moderate-to-high heat for 2 minutes each side, or until lightly browned.
Return the bacon to the frying pan. Add the sauce and stir, scraping up any residue from the bottom of the frying pan. Simmer for 3–4 minutes, or until the liver is cooked but still slightly pink in the centre.
Stir the sour cream into the frying pan and season to taste with pepper (because of the bacon, no salt should be needed). Simmer for 1–2 minutes to heat the cream.
Transfer to a warmed serving dish, garnish with the small sage leaves, if using, and serve.