4 tablespoons extra virgin olive oil
4 tablespoons unsalted butter
8 onions, very thinly sliced
500 gram calves liver, thinly sliced
1/3 cup red wine
A few drops of Balsamic vinegar
2 tablespoons passata tomato puree.
Heat the olive oil with 3 tablespoons of the butter in a large, heavy sauté pan over a medium flame. Add the onions with a few pinches of salt and sugar and cook them over low heat until very soft, slightly golden but not too dark. Remove them to a warm platter.
Dust the liver with seasoned flour, shaking off any excess flour.
Add more olive oil and butter to the sauté pan and add the liver, cooking for 30 to 45 seconds on each side. Work in batches if necessary to avoid overcrowding the pan. When done, place the liver over the onions and keep warm.
Add the wine to the pan and scrape the bottom of the pan with a wooden spoon to dislodge any browned bits. Add the tomato puree, balsamic vinegar and cook down until reduced to a smooth sauce. Pour it over the liver and onions. Serve immediately.