Finely chop the onions, reserving about 2 tablespoons for later. Warm the oil in a frying pan over a low to medium heat. Add the garlic and spices and cook and stir for a few minutes. After a couple of minutes add the onion and gently cook for 15 minutes, stirring occasionally. Add sugar, flour and gradually add water, stirring. Simmer for 15 minutes, then set aside.
Cut chicken into bite sized chunks and fry in batches until cooked through, then set aside on a plate. Once onion mixture has cooled, blitz with a hand blender until smooth, season with salt and pepper, and add cooked chicken. Stir 3 to 4 tablespoons of the korma paste and stir through.
Add the remaining onions and cream. If the sauce is not thick enough you can add more flour, but be aware once you heat this mixture it will thicken quickly so be sure to stir. I always make ahead so I can just warm up on the evening.