Finely chop the onion and place in heavy bottomed pan, drizzle in some olive oil and place on a medium heat. Add in the garlic and stir, leave for a minutes and add in all the rice. Stir the rice until it is all covered in oil and has gone slightly opaque. This should take about a minute and add white wine and stirr. After a minute pour half of stock into the pan and stir, from this point it will take 18-20 minutes to cook the rice. After 3 minutes pour the other half of the stock. You will need to keep stirring the risotto every couple of minutes adding in more stock whenever it starts to look dry. After about 10 minutes when the rice is almost cooked add in the asparagus and allow cooking for 2 minutes. After two minutes taste the rice, if it is cooked then move to the next step, if not keep adding water and stirring until cooked. Once the rice is cooked add the prawns, cherry tomatoes, parmesan, and butter to the pan. Stir continuously for a minute and a half till the prawns and asparagus are cooked, season with salt and pepper, remove from the heat and serve.