Peel the onions and finely slice. Peel and finely chop or grate the garlic and ginger. Halve the chilli, flicking out the seeds and white pith for less heat. Finely chop the chilli. Roughly chop the tomatoes.
Slice the roots off the celeriac on the diagonal, leaving as much flesh behind as you possible. Peel the celeriac and chop it into bite-size chunks. Drain and rinse the chickpeas.
Pour 2 tbsp oil into a large pan and warm over a medium heat. Add the onions, garlic, ginger and chilli. Sprinkle in 1-2 tbsp tikka spice mix (add more or less, depending on how strong you want the flavour to be), and add a pinch of salt and pepper. Stir well and fry for 5 mins, till everything has slightly softened.
Stir in the chickpeas, celeriac and tomatoes. Tip in the dried lentils. Crumble in the stock cube and coconut cream. Pour in 1 ltr boiling water and stir to combine. Bring to the boil, then reduce the temperature to a simmer and cover the pan with a lid. Cook gently for 30 mins.
Meanwhile, trim any woody ends off the cavolo nero and shred the leaves. When the curry has cooked for 30 mins, stir in the cavolo nero and cook for a further 5 mins, till wilted.
Have a taste of the curry and add a little more salt and pepper if needed. Ladle into bowls and serve.