Preheat oven to 180°C. Parboil the peas, following the instructions on the package. Drain thoroughly and let cool.
Cook the rice al dente and put into a bowl after draining thoroughly. Sprinkle on some grated nutmeg and add the cold milk.
Beat the eggs with the Parmigiano and add a pinch of salt. Add to the rice. Add some diced ham and the peas and mix well. Butter some muffin tins and fill each one half-way. Level with a fork and arrange the cubes of Mozzarella. Complete by pouring in the remaining rice mixture.
Add some bits of butter on top and bake for 30 minutes or until the surface of the tart is golden brown.
Remove from oven and serve hot.