Peel and devein prawns, leaving tails intact. Place prawn heads and shells in a medium saucepan with vegetable stock. Bring to boil, cover with lid and allow to simmer for 20 minutes. Strain stock, and return liquid to pan. Bring stock back to boil, cover with lid and continue to simmer while preparing risotto.
Melt half the butter in a deep frying pan or large casserole. Add onion and cook, stirring, until soft. Add rice and stir to coat well in butter mixture. Add enough hot stock to just cover rice and cook, stirring occasionally until absorbed. Repeat process with stock, for 15 minutes. Add tomatoes and asparagus, and continue to repeat process with stock, for a further 5 minutes. Add prawns and remaining stock, and cook for another 5 minutes or until rice is just tender and prawns are cooked.
Stir through remaining butter, parmesan and most of the basil. Season to taste. Serve immediately topped with remaining basil .