Heat the oil in a medium-sized nonstick saucepan over moderate heat. Add the onions, spring onions and garlic. Cover and cook for 7 minutes or until the onions are soft. Uncover and cook, stirring occasionally, for 5 minutes.
Stir in the orzo. Add the stock and 1 cup water. Bring to the boil, cover and simmer for 20 minutes. Stir in the Parmesan.