In a large saucepan, bring the stock to a simmer and keep it warm over low heat.
In a separate pan, melt the butter over medium heat. Add the onion and sauté for 2-3 minutes until translucent.
Add the garlic and mushrooms, and continue cooking for 5 minutes until the mushrooms have released their liquid.
Add the Arborio rice to the pan and stir to coat the grains with the butter. Pour in the white wine and cook, stirring constantly, until the wine has been absorbed.
Ladle in the warm stock, one cup at a time, stirring constantly until the liquid has been absorbed. Repeat this process until the rice is tender and creamy, about 18-20 minutes.
Stir in the prawns and Parmesan cheese, and season with salt and pepper to taste.
Serve the creamy prawn and mushroom risotto immediately, garnished with additional Parmesan if desired.