Sweat the garlic and 4 shallots in ½ of the butter. Add the rice and fry, stirring all the time until the grains of rice are glassy. Stir in thyme, saffron and ⅔ Prosecco, boil till it has absorbed then add half of the stock, a little at a time. Keep adding the stock one ladle at a time, mixing continually with a wooden spoon. Beat in cold butter a little at a time then season with parmesan, and salt and pepper. Trim the mushrooms, wash and dry. Sweat the other 2 shallots in remaining butter until transparent. Add the mushrooms and fry for 1-2 mins. Season with a little salt and pepper and add to risotto. Finish with whipped cream and remaining Prosecco.