Heat the oil in a frying pan, then sauté the onion for about 5 minutes until soft. Add the potatoes and 375ml vegetable stock, and cook until the potatoes are tender.
Add the fish and half the corn and warm gently until the fish is cooked through. Remove from the heat and purée in a blender. Return to the pan and stir in the rest of the corn, stock and the milk. Heat until the chowder is warmed through, being careful not to let it boil. Serve.