1) Melt the butter in a saucepan, add the onion and garlic, cover with a lid and cook for 5 minutes. Pour the chicken stock into a separate pan and bring to a simmer.
2) Add the rice to the onions and coat in the butter. Add the white wine, boil and reduce for 2 minutes. Add just enough hot stock to cover the rice.
3) Heat the olive oil in a separate frying pan and fry the mushrooms for 3 minutes until golden brown. Season and add to the rice. Cook the rice and mushrooms for 10 minutes, adding a little more stock once the previous batch has been absorbed by the rice.
4) When the rice has softened but still has a slight bite, add the mascarpone, Parmesan, parsley and lemon juice.