To make the risotto, fry the onion in the truffle butter in a large frying pan.
When the onion is soft, add the garlic and fry for another 20 seconds or so then add the mushrooms. Fry until the mushrooms are golden brown then add the rice.
Stir to coat the rice in the butter and aromatics then add the wine. Allow the wine to reduce then stir in the stock, ladle by ladle. Allow the rice to absorb the stock before adding more and stir continuously. It is this continuous stirring that creates the creaminess risotto is known for.
When the risotto has absorbed approximately half of the liquid, start tasting it regularly as a good risotto can quickly be ruined by over cooking. Your risotto might also not need all the stock or it might need more, it all depends on the rice's absorbency.
When the risotto is cooked to your preference, stir in the butter and Parmesan and season to taste with lemon, salt and pepper.
Top with the fresh black truffle and serve.
Notes
I am aware that not everyone has the luxury of cooking with fresh truffles. You can substitute all the truffles in this recipe with 1-2 teaspoons of black truffle oil. Buy your truffle oil from a reputable source as often the oil is simply infused with truffle essence and not real truffles.