Bring the water to a boil (a kettle works well for this).
Quarter the onions and then break them apart into petals (roughly – two or three petals per quarter)
Combine all ingredients except tomatoes, bring to a gentle boil and simmer, loosely covered, for one hour. Use a big pot!
Add tomatoes, stir and simmer an additional 20 minutes
Let cool slightly. Blend to smooth consistency. Make sure you remove the centre cap from the blender lid and cover the hole with a cloth or you will be cleaning the ceiling. Alternately you can use an immersion blender.
Wipe out the pot and return pureed curry base and simmer, loosely covered, until the oil separates out – this can take an hour or more. Stir the oil back into the base. At this point you can portion out the base into 2 cup portions and freeze if desired.