Heat one tablespoon of the oil in a small, non-reactive frying pan over a medium heat. Add the button onions or shallots and fry all over for 5-6 minutes, or until softened and pale golden-brown. Remove from the pan and set aside.
Add another tablespoon of oil to the pan the onions were fried in. Add the lardons and mushrooms and fry for 4-5 minutes, or until pale golden-brown. Remove from the pan and set aside with the onions.
Add the remaining tablespoon of oil to the same pan. When the oil is hot, add the kidneys, cut-sides down, and fry for 3-4 minutes on each side, or until golden-brown and just cooked through.
Remove from the pan and set aside with the onions, bacon and mushrooms.
Add the red wine vinegar to the pan. When it simmers, scrape up any burned bits from the bottom of the pan using a wooden spoon to deglaze the pan, then add the gravy.
Return the kidneys, onions, bacon and mushrooms to the pan and bring the mixture to a simmer. Continue to simmer until the kidneys and vegetables have warmed through, then stir in the herbs.
To serve, place the toast into a shallow serving bowl. Spoon over the pan-fried lambs' kidneys, vegetables and gravy.
