Peel, halve and slice the onion. Roughly chop the remaining vegetables.
Heat the oil in a large pan and fry all the vegetables until soft and lightly browned. Pour in the water.
Bring to a boil, skim the surface then lower the heat, partially cover the pan and let simmer for 1½ hours. Strain the stock and let cool. Store in refrigerator for 2-3 days.