Prepare grill to medium heat.
Drizzle olive oil over peppers and place peppers over direct heat on the grill.
After about 6 minutes turn the peppers over.
The peppers should be nicely charred. Grill for another 6-10 minutes.
Cover peppers in a container and let sit for 10 minutes.
Remove skin from charred peppers.
Remove membranes and seeds from some of the peppers and roughly chop.
Add peppers to a pan and add chopped garlic and onions.
Add water, vinegar, salt and coriander.
Simmer for 45 minutes over medium heat stirring occasionally.
Puree mixture until it is no longer chunky.