Melt the butter in a large pan over medium heat, add the onion and celery and saute until tender and soft. Stir in the rice and season with salt and pepper and continue to cook for 1 minute. Add wine and stir until almost all liquid is absorbed, about 1 minute. Add a ladleful of the hot broth and cook, stirring, until it has been absorbed. Continue adding the broth, a ladle at the time, and stirring until each addition has been absorbed. This will take about 20 minutes. Stir in 1/2 cup Parmesan cheese. Allow to cool and then put in the refrigerator, covered, overnight.
Separate 1 egg and reserve the yolk.
In a big mixing bowl, add the cold risotto, egg yolk, remaining 1/3 cup Parmesan, ham, mozzarella cheese, and parsley. Stir well, using a wooden spoon, until all the ingredients are well mixed.
Whisk two eggs in a medium bowl.
Place the flour in a plate or small bowl and the bread crumbs in a separate plate or small bowl.
Roll, roll, roll your risotto into balls.
One-by-one, gently roll each ball into the flour, then dip it into the beaten eggs, and then roll it into the breadcrumbs. By doing this you will create a nice crust on the outside of the croquettes, it will help maintain its round shape while cooking, but it will also keep the warmth inside for a while after they are cooked.
Heat up about 1 1/2 cups vegetable oil in a high-sided skillet.
When the oil is hot, start frying the croquettes. Try to remember the order in which you put them into the pan, the first one into the hot oil should be the first one to be turned over, and then the first one to come out. Keep it in mind as you cook!
The process is fairly quick, turn the croquettes a few times to make sure they are evenly cooked, and do not worry if some mozzarella cheese spills out. When it happens, if it happens, there is nothing you can do about it, and the dish won't be affected in any way.
Remove the risotto balls to paper towel-lined plates.
Allow to cool just slightly before serving...especially if you are serving it to kids, they can really get hurt. This recipe works great with risotto leftovers, especially vegetable-based ones. In general though, I would not make croquettes with seafood risotto...I usually do not save leftovers.