To make the risotto, fry the onion in the a splash of olive oil 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 oil 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 stirring 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, truffle oil (if using), salt and pepper.
Half way through making the risotto, heat a large pan over medium-high heat and add a splash of olive oil. Season the pork fillet then add to the pan and allow to brown all over. Add the garlic and rosemary and allow to cook until the pork is cooked to your preference. Remove the pork from the pan and allow to rest for 5-10 minutes. • Slice into thick slices and serve with the risotto.