Bring the chicken stock to boil in a saucepan and reduce heat to a simmer. In another saucepan, heat the butter and half the oil, add the onions and cook until soft, stirring all the while. Add the rice, and stir over medium heat until the rice is coated and oily. Add the wine and stir until absorbed.
Add the stock a little at a time while stirring, making sure the liquid is absorbed before adding more. When the rice is tender, add the asparagus, fresh herbs, lemon zest and cheese and stir until the asparagus is cooked but still crisp.
Meanwhile, heat the remaining oil in a frypan and cook the prawns until just pink. Stir them through the risotto and season with salt and pepper to taste. Serve immediately.