To make the relish, put the onions, juniper and thyme in a large saucepan, season with 1 tsp salt and a little pepper. Cook on a high heat for 10 mins, stirring regularly as the onions start to soften. Add the vinegar, sugar and 100ml water. Cover, boil for 5 mins, then uncover and simmer gently for 5 mins or until soft and with a little juice remaining. Meanwhile, remove large tubes, membrane or gristle from the liver. Pat dry with kitchen paper.
Heat 1 tsp oil in a large frying pan and add the prosciutto. Fry for 1 min each side or until golden and crisp. Set aside on a plate and keep warm in a low oven.
Add a splash more oil to the pan. Dust the liver slices in the flour and pat away any excess. Place the liver in the hot pan, scatter the sage in, then fry for 1 min each side if the liver is sliced thinly, longer if thicker. When the liver is golden and just pink in the middle, transfer it and the crisped sage to the plate in the oven.
Return the pan to the hob and add the Marsala. Boil until syrupy. Let the butter melt into this sauce, season to taste, then set aside. If your sauce reduces too quickly, just add a little water. Plate up the liver, prosciutto, sage and warm onion relish, then spoon over the sauce. Serve with seasonal greens and potatoes, if you like.