In your slow cooker, add all ingredients listed under "to slow cook". Give a stir.
Slow cook ingredients for 8 hours.
In the morning, this is how your chickpeas will look. All done and soft. Keep it warm while you make the tempering.
Heat oil in a pan. Once hot, add chopped ginger. Saute few seconds till ginger starts turning light golden in color.
Add chopped onion, cook for 2-3 mins till raw smell goes away.
Add chopped tomatoes and salt. Mix and cook for 5 minutes.
Add red chili powder, garam masala, chole masala, cumin powder and mix.
Cook for another 6-7 minutes till tomatoes are nicely done and oil starts oozing out from the sides. [you may add 1 tablespoon of water if you think spices will burn while the tomatoes are cooking].
Transfer cooked masala to slow cooker. Add kasuri methi and 1/4 cup water (if required), Mix everything.
Slow cook again for 2 hours.
Add in chopped cilantro and mix. Serve chana masala with rice or any bread of your choice.