Heat the oil in a large saucepan or casserole, and cook the onion and garlic gently for 7–8 minutes, until softened. Stir in the rice and cook for 1 minute, then pour in the chicken stock. Simmer gently for 12–15 minutes, until the rice is just tender. Remove from the heat.
In a small bowl, whisk the lemon juice with the beaten eggs and a pinch of salt. Continue whisking while you add a ladleful of hot stock in a slow, steady stream, then whisk the egg mixture into the saucepan of soup. Return the pan to a very low heat, and continue stirring for 2–3 minutes, until the soup has thickened slightly, taking care not to allow it to boil.
Stir in the chopped spinach and season to taste, then ladle the soup into 4 bowls. Serve scattered with chopped parsley and grated cheese.