Posta negra is beef that is seasoned, browned and roasted, served with a black sauce made with the pan drippings. At its best when cooked medium-rare and very thinly sliced across the grain, it is delicious in sandwiches.
Mix the onions with the thyme, garlic, mustard, cloves and black pepper. Rub over the meat. Set aside for at least 1 hour, or overnight, in the refrigerator. You can leave it for up to 4 days to improve the flavour and tenderness.