Add the halved garlic head, cut side down, pushing it to the bottom of the pan. Add the tomato purée and heat for a minute or two to cook it out. Pour in the wine to deglaze the pan, scraping up the bits at the bottom. Bring to a boil and cook for 10–15 minutes until the liquid is reduced by half, then add stock to nearly cover the ribs (you’ll need less stock if your roasting pan isn’t very large). Bring to a boil again, basting the ribs with the juices.