Place the stock in a large saucepan and bring to the boil over a high heat; reduce heat and keep at simmering point.
Place a large heavy-based saucepan over a medium heat and add the olive oil, onion, salt, and half the butter; stir until the onions are translucent, add the rice and stir for 1 to 2 minutes, or until the rice is well coated.
Gradually add all the simmering stock, a cupful at a time, stirring constantly and making sure the stock is absorbed before you add more; this should take 20 minutes and the rice should be al dente and creamy.
Remove the saucepan from the heat, and then stir in the remaining butter, lemon zest, lemon juice and pepper to taste; cover the saucepan and let sit for 3 minutes for the flavours to develop.
Meanwhile, pound the chilli and garlic in a mortar and pestle; place the extra olive oil in a frying pan over a high heat and heat until hot.
Season the prawns with salt and pepper, cook for 2 minutes, shaking the pan, until the prawns are just opaque then add the chilli and garlic and cook for 1 minute; remove from the heat and set aside.
Stir the risotto and divide between four bowls; toss the prawns with the parsley then place on top of the risotto, serve with lemon wedges.