Wash then slice up all the vegetables, but not too finely.
Heat the oil in the pressure cooker then fry the onions, carrots, leek, celery & mushrooms for about five minutes over a lowish flame for about 5 minutes.
Add all the other vegetables (including the tomato tin & purée) plus the stock and bring to the boil.
Tip in the pasta, herbs & seasoning, bring up to pressure and cook for 5 minutes then allow to depressurise by itself. Or if using a normal pan just simmer for about 20 minutes stirring regularly to avoid sticking.