Chop all the vegetables into smallish cubes.
Spray the pressure cooker with the oil. Heat gently and add the leeks, mushrooms, pepper & tomatoes. Allow to sweat for a few minutes then add the other chopped vegetables.
Stir occasionally for another couple of minutes to let the juices come out of the vegetables then pour in the can of tomatoes. Dissolve the stock cube in the water and stir into the pan.
Cook under pressure (8-9 mins high,11-12 medium) according to the maker's manual. Take off heat and allow to cool by itself.