Put the stock and water in a saucepan and bring to a simmer over medium heat. Heat the butter and olive oil in a large stock pot or Dutch oven. Saute the onion for several minutes until translucent. Add the rice and saute for a few more minutes. Pour in the Sherry or wine and cook until the liquid is absorbed. Then add the broth, about 1/2 cup at a time, and stir the rice until the liquid is absorbed between each addition. Keep adding the broth and stirring for about 30 minutes. Test the rice for doneness. Mine usually takes about 30 to 40 minutes, and uses every drop of broth. When you add your last bit of broth, add the lemon juice and herbs as well. When the risotto is done, take it off the heat and stir in the mascarpone and Parmesan cheeses. Season with salt and lots of cracked pepper and serve with more grated Parmesan on the side.
Other risottos to try:
Lemon Risotto with Grilled Asparagus
Pumpkin Risotto with Browned Butter and Sage
Spring Vegetable Risotto