Place the stock in a saucepan, bring to the boil, then reduce heat to low and keep stock at a simmer.
Meanwhile, heat the butter and olive oil in a deep frypan over medium heat, add the onion and cook, stirring, for 1-2 minutes until softened, but not coloured. Stir in the garlic, then add the rice and cook, stirring, for 1 minute to coat the grains. Add the wine and allow the liquid to evaporate. Add the stock, a ladleful at a time, stirring occasionally, allowing each ladleful to be absorbed before adding the next. Continue until you have one ladleful of stock left - this should take about 20 minutes. Add the prawns with the final ladleful of stock and continue to stir for 2-3 minutes until the prawns are cooked through. Stir in the parmesan, then cover and remove from the heat.
Peel the avocados, dice and toss in the lemon juice. Just before serving, carefully stir the avocado mixture into the risotto. Serve in bowls topped with extra parmesan and chopped chives.