Pour the stock into a saucepan. Bring to the boil, then reduce the heat and keep at a low simmer.
Heat the oil and half the butter in a large wide heavy-based saucepan and cook the onion and pancetta over low heat for 5 minutes, or until softened. Stir in the parsley and peas and add two ladlefuls of the hot stock. Simmer for 6–8 minutes.