1) Put the rapeseed oil and coconut oil in a saucepan over a medium heat.
2) Add the mustard seeds and fry until they start popping.
3) Add the onions and fry for 5 minutes until they are soft stirring regularly.
4) Stir in the garlic and the rest of the spices and fry for a few minutes.
5) Crumble in the stock cube then add the tinned tomatoes and salt and pepper to taste.
6) Add enough water to fully cover the onions, bring to a simmer then cover and cook for 20 minutes.
7) Use a stick blender to blitz the sauce until it is smooth.
Add vegetables, meat or fish to the sauce after blitzing
To give the curry a South Indian flavour add 45g grated creamed coconut.
Try adding a big knob of butter to turn it into a rich buttery sauce.
Add crushed peanuts and coconut cream for a Malaysian flavour.
Add raisins for a fruity flavour.