1. Bring a large pan of salted water to a boil, and blanch the green beans, asparagus and peas for 3-4 minutes before draining in a colander. Set aside.
2. In a large casserole or saucepan, fry the onion and garlic in the oil for 3-4 minutes until soft.
3. Pour the rice and vegetable stock into the saucepan and stir-fry for 2-3 minutes until the grains are cooked. Combine the blanched vegetables followed by the Parmesan cheese and mascarpone to make the dish creamy.
4. Season to taste and serve immediately.