Heat the butter in a large saucepan over a medium-low heat and tip in the onions, garlic and ginger and fry for 12-15 mins until soft but not coloured. Stir in the curry powder, turmeric, raisins, vinegar and star anise, and fry for 1 min more, then pour in the chicken stock, bring to the boil and simmer very gently for 30 mins, stirring so it doesn’t catch on the bottom.
Lift out the star anise, mix the cornflour with 2 tbsp water and stir into the sauce. Simmer for 5 mins, stirring constantly until you have a thickened silky sauce. Pour into a blender and blitz until smooth, or use a hand blender. Squeeze in lemon juice to taste. Use this sauce to make a curry for your Christmas leftovers whether its sprouts, turkey, or simple roast potatoes. If freezing the sauce, leave to cool, pour into a container and freeze, then defrost and reheat when you’re ready to eat.