Soak the ham hock in cold water for a few hours or preferably overnight.
Drain and place it in a saucepan. Cover with cold water, bring it to the boil, then reduce the heat and simmer for 1 hour 30 minutes to 2 hours. Drain again, and set the ham aside.
To make the chilli oil, place the groundnut oil in a pan over a very low heat. Peel the garlic and finely slice along with the chillies, then add to the pan and warm through for 5 minutes, making sure the garlic doesn’t brown. Set aside to steep while making the soup (the oil can be strained before using, or left as is).
For the soup, peel and roughly chop the onion and ginger (keep them separate), then scrub and roughly chop the parsnips.
Heat a glug of olive oil in a large pan and add the onion. Cook gently, stirring often, for 5 minutes, then add the parsnips, rogan josh paste and ginger, and cook for a further 5 minutes.
Add the lentils, stock (if using) or the same amount of hot water and the ham hock and bring to the boil. Simmer for about 30 minutes, until the lentils are nice and soft.
Remove and discard the ham bone and liquidise the soup until smooth. Return any lean ham to the saucepan and reheat.
Finely shred the mint leaves and serve scattered on top of the soup with a dollop of yoghurt and a swirl of the chilli oil.