Soak the lentils and kidney beans overnight. While it is not necessary when using a slow cooker, I find soaked legumes make for better digestion. There is a worry that slow cooker does not cook the kidney beans completely. It's said that undercooked kidney beans is toxic. I usually add boiling hot water to soak the legumes and have found it to be well cooked when using the slow cooker. But if it still concerns you, boil the kidney beans for 10 minutes before adding it to the slow cooker. Or simply use canned kidney beans towards the end of cooking.
Drain the soaked lentil mixture and add it to the slow cooker. You can coat the slow cooker insert with oil but its optional.
and set the slow cooker to setting of your choice. When I am cooking overnight, I cook in low (8-10 hrs), but if its during the day (say around noon for dinner), I choose the High (4-5 hours) option.
Add turmeric, bay leaf, ginger and garlic. These help in digestion as well reduced or no flatulence that usually follows consuming legumes.
Close the lid. For this post, I was doing it during lunch time for dinner. Hence used High setting
This is how it looks around 3.45 hours. [When cooked Low for 8 hours, it's always completely cooked] The lentils are completely cooked and kidney beans 99% cooked with a very slight bite to it. I continue cooking it.
You can choose to mash it with a spatula, but I just run my hand processor few times to mash it a bit.
That way it also helps to thicken up the dal. See its thick with a nice texture to it. Close the lid and continue cooking.
While that's happening, I cook the aromatics on the side. In a hot pan, add required amount of Ghee/butter. This is traditional but you can choose to just add a little bit of oil as well. Add the cumin seeds and once it starts spluttering, add the chopped onion along with the chillies.
Once soft - around 1 minute mark, add the chopped tomatoes.
[Tip: To save yourself even more time, you can choose to saute onions very lightly right in the beginning and add it along with the dal for cooking. Then in the last one hour of cooking, I would add 1 cup of tomato puree to the dal. I find adding acidic ingredient right in the beginning, does not cook the dal well and it get tough while cooking. Hence I would add salt and tomato towards the end of cooking]
Saute for another minute till it gets soft and then add all the spices along with salt.
Cook for a minute or so until the tomatoes break down. Can add few teaspoons of water to avoid the spices burning. This total sauteeing of onion and tomatoes should not take more 5-6 minutes in med-high heat.
Add this mixture to the dal
and give it a good stir. Taste and adjust seasoning if needed and let it cook for another 30minutes or so. If you are in a time crunch, you can keep the setting on high and cook for another few minutes. But if you still have time, let it simmer in low until serving time. I find that the more it cooks, better the flavor and texture.