Slice the onion finely. Heat the oil in a heavy pan, add the onions and cook slowly over gentle heat until they are soft and sweet. Remove the onions, increase the heat and add the mince. Keep stirring until the meat is taking on a caramel colour in places. After five minutes, add the finely chopped garlic and continue to cook for a few more minutes. Add back the cooked onion, the tinned tomatoes, tomato paste, Madeira, port and lemon zest. Tie the thyme into a bunch with string and add to the pan attached to the saucepan handle. Season generously and simmer for 20-30 minutes until the liquid is well reduced.