Heat the oil in a flameproof casserole dish over a medium heat and cook the onion with a pinch of salt for 8-10 mins. Add the garlic and cook for another minute, then stir in the chipotle paste, cumin, 1 tsp paprika, the oregano and tomato purée. Cook for 3-4 mins until dark, then transfer half the mixture to a saucepan.
