Warm the oil in a heavy based pan that will hold everything- a casserole or a 22cm diameter saucepan will be big enough. Fry the pancetta, stirring, until it becomes crisp and browned then add the peas and stir for a minute until thawed.
Add the pasta and stir well with the pancetta and peas then pour in the boiling water. Add a little salt then turn down the heat and leave to simmer for 10 minutes, checking and stirring once or twice and adding a little more boiling water if needed.
When it is ready the pasta should be soft and the water absorbed. Beat the butter and parmesan into the pan, check the seasoning and serve immediately in warm bowls.