Remove all the membrane from the kidney and soak the kidney in 2 cups of water with 3 tablespoons of vinegar for 2 hours. Then cut it into thin slices and dredge these in flour seasoned with salt and freshly ground black pepper. Heat butter and olive oil and sauté the kidney slices quickly, browning them on both sides. Add finely chopped garlic and onion, and continue cooking for 5 minutes. Then add the rosemary, the bay leaf and the red wine. Simmer for 15 minutes or until the kidney slices are tender. Serve with boiled potatoes.