Puree the onions, tomatoes, garlic and ginger. Drain and rinse chickpeas.
Place in the crock pot along with the rest of the ingredients, except the salt and the coriander leaves.
Add 3 cups water and set the slow cooker on high (it is important to set this on high to ensure the raw ingredients like the onions and tomatoes cook thoroughly). Add more water if the mixture dries up.
Let the chana masala cook for six hours. Check occasionally and add water if needed.
After six hours, add salt and coriander leaves and mix well.