Melt the butter in a large heavy-based saucepan over a moderate heat. Add the onion and sauté for 3 minutes until just soft, then stir in the rice and sauté for 2 minutes until the rice becomes translucent.
Pour in the wine and cook for 3 minutes until the wine has reduced by half. Pour in 1/2 cup of the stock, stirring almost continuously, gradually adding more of the stock as it is absorbed, until you've used about 2 cups of the stock.
Stir in the asparagus stems and another 1/2 cup of stock and cook, stirring constantly, until the stock is absorbed. Add the zucchini, asparagus tips and another measure of stock.
Continue cooking, stirring constantly and gradually adding the remaining stock. Add the peas with the last measure. By the time all the stock is absorbed, the rice should be creamy and tender but still firm. Remove from the heat and stir in the parmesan. Season, then serve, sprinkled with parsley.