While the chicken is marinating, make the sauce. Heat the butter in a casserole dish or sturdy saucepan with a lid. Add the onions along with a pinch of salt and fry cook 10 mins, until golden. Add the reserved paste and continue to cook for 3-4 mins until starting to colour. Stir in the tomato purée and cook for another 3-4 mins until everything is sticking together, then add the chopped tomatoes, 2 cans of water and the stock pot. Simmer for 20 mins to make a thick, glossy sauce. Leave to cool, then cover and keep chilled. Will keep chilled for up to three days, or frozen for three months.