Bring 2 cups water and all of the stock to a boil, then reduce heat to low.
In a large skillet, heat oil and butter over medium to medium-high heat. Add the onions and sauté for 3 minutes. Add Arborio rice and sauté, 2 to 3 minutes or more. Add wine or sherry and allow liquid to absorb for 1 minute. Add half the stock or broth and reduce heat slightly. Simmer, stirring frequently, until liquid is absorbed, then add more liquid a few ladles at a time. As liquid cooks out, ladle in more.
When risotto has cooked almost to al dente, about 18 minutes, fold in spinach, basil, and parsley. Season with nutmeg, salt, and pepper and stir in any remaining broth. Stir in cheese when risotto is done cooking and serve immediately.