Heat the oil in a large pot, add the onion and stir frequently until the onion is soft, transparent but not brown. Add the tomatoes, half the coriander and the stock, and simmer for about 15 minutes. 
Put everything in a blender or use a hand blender and blend until very smooth and silky. 
Garnish with lemon zest and fresh coriander.

A simple and quick way to peel tomatoes:
plunge them into boiling water for a few seconds, then plunge the tomatoes into ice water. Peel away the skin.