Place the oil and onion into a large pan. Cook on medium heat for 5 minutes, stir occasionally. Add the rice to the pan and stir to coat the rice with the oil. Toast for 1-2 minutes.
Pour the wine into the pan. Scrape up any brown bits. Bring the wine to a boil and cook for 2 minutes. Increase the heat to medium-high.
Pour 3 cups of chicken stock into a measuring cup or bowl. Add a few ladlefuls of the stock into the pan. Stir the rice very frequently to prevent the rice from sticking to the pan. Once the chicken stock has been absorbed by the rice, add another few ladlefuls of stock, stirring very frequently. Continue this process until 3 cups of the chicken stock is used and the rice is cooked.
Pour the remaining three cups of chicken stock into the pan once the rice is cooked. Reduce the heat to low and warm through for five minutes. Stir in the butter, saffron, salt, lemon juice and lemon zest. Stir well.
Once the butter is completely melted and mixed into the soup add the parmesan and mascarpone cheese. Continue to stir until both are melted and incorporated into the soup. Remove from the heat. Add the parsley (reserving a 1/2 cup to top individual bowls of soup) and stir.
Place the soup into individual bowls. Top with a dollop of mascarpone and chopped parsley. Serve immediately.
