Mince the leek and put the oil and the butter in a pan. Add the leek and cook slightly. Add the rice and increase the heat to let the rice toast for a while. Add the spumante and let it evaporate. Add a little of the stock, lower the heat and keep cooking adding the remaining stock, without allowing the rice to dry. When the rice is ready, take it off the heat, add the parmesan cheese, cover and let it rest for 2-3 minutes, then add the green pepper and serve.