Tail the sprouts, remove any blackened, spotted leaves, then cut the large ones into quarters and halve the small ones.
Put the mustard, cumin, coriander and fennel seeds in a mortar, bash as finely as you can, then stir in the nigella seeds.
Heat two tablespoons of oil in a large frying pan for which you have a lid. When hot, add the onion and fry, stirring, for five minutes. Add the spice mix, and cook for another five minutes, until the onions are soft and browning. Stir in the ginger, garlic and chilli, and cook for three minutes more.
Add the tomatoes, tomato puree and a teaspoon and a half of salt, stir to combine, then cook for 15 minutes. Add 400ml water and the sprouts, pop on the lid and leave to simmer for 10-12 minutes, or until the sprouts are just tender. Take off the heat, season with lemon juice and serve.