Peel and very finely slice the red onion. Place in a large non-stick frying pan on a medium-high heat with 1 teaspoon of olive oil.
Reserving 2 nice leaves, pick, finely slice and add the rest of the sage. Cook for 5 minutes, tossing regularly.
Push the onions to one side, and add the bread and bacon to the pan. As soon as they crisp up, flip them over. Drizzle the soft, golden onions with a little red wine vinegar, toss with tongs, then move them on top of the bread, so they don’t catch. Once the bacon is golden, move that on top of the onions.
Lightly season the liver, then add to the pan to sear for just 1 minute on each side, so it’s golden on the outside, blushing in the middle.
Add the 2 reserved sage leaves alongside, with 1 teaspoon of olive oil to crisp them up, then serve.