Hi andy,
I cook chicken Dhansak often and I have found that red lentils work well, but yellow is also ok. I always boil then simmer the lentils for about 25-30 mins they should breakdown, let them cool down for a while then put them in a blender and blend until smooth. put into a container then into the fridge. you can do this a few hours before of the day before if you want.
when you come to use it you will see the lentil mix will be firm and hard but that's good just spoon into the pan when cooking and mix into the sauce.
I love Dhansak and I have good results now after lots of pactice .
Bob