Heat the butter and olive oil in a heavy frypan. Add onion and saute until soft.
Add the rice and stir to coat with oil until rice is translucent. Add a splash of wine and cook until evaporated. Add garlic.
Add hot stock, a ladleful at a time, and cook until absorbed, stirring constantly. It should take about 20 minutes to cook the rice. Add prawns when you estimate you have about five minutes left, and cook until pink.