First of all prepare the Courgettes. Trim off the ends, then cut off the skin in thick strips (1/4 inch thick) and discard the white flesh, which tends to be mushy. Then chop the green parts of the courgettes very finely and set aside.
Place the stock in a saucepan and bring it to a gentle simmer and leave over a low heat. Heat the olive oil in a medium sized heavy based saucepan. Add the celery and leek and sweat until softened. Add the rice and stir until each grain is coated with oil. You will notice the rice becoming shiny. At this stage, add the wine and keep stirring until it evaporates. Then add the peas, beans and courgettes and mix well, making sure that the vegetables do not stick to the pan. Add a couple of ladlefuls of the stock and cook, stirring all the time, until it has been absorbed.Repeat with more stock. Continue adding the stock in this way until the rice is cooked, which usually takes about 20 minutes. To check if the rice is done, taste it and it should be soft on the outside but al dente inside.
Remove the pan from the heat and beat in the butter and Parmesan with a wooden spoon. Taste and adjust the seasoning. Leave to rest for 1 minute, then serve.