Place the onion and garlic in a small food processor and whizz until smooth, adding a little water to help it along if necessary.
Place the coconut milk in a pan with a heavy base and bring it to the boil. Reduce the heat and cook, stirring occasionally, until the milk has reduced by half. Be careful not to let the coconut milk brown or stick to the pan.
Grind the fennel seeds and cloves using a pestle and mortar and add to the pan along with the coriander, cumin and salt and stir for 1 minute. Add the meat and cook for a couple of minutes until it changes colour. Add the onion and garlic mixture, chilli, lemongrass, lemon juice and sugar and mix well.
Bring it back to the boil, cover and reduce the heat right down. Cook for about 1 and a half to 2 hours until the beef is cooked and the liquid is reduced to quite a thick sauce. You will need to check the curry and stir it frequently throughout the cooking to prevent it sticking to the bottom of the pan. Add a little more coconut milk if necessary.
For the last 10 minutes of cooking, simmer with the lid off, stirring all the time. The curry is ready when it is brown and quite dry.