Peel and cut the potatoes in quarters. Place in a large pot, cover with water, add salt and bring to a boil over medium-high heat (about five to seven minutes). Lower to medium heat and simmer until tender, about eight to 12 more minutes, based on the size of the potatoes. Check with a fork to see if tender all the way through.
Drain and carefully pass the potatoes through a ricer when still hot. Collect the mashed potatoes in a bowl. Let them cool for five minutes. Taste the mashed potatoes and adjust with salt if necessary
Add one egg, the shredded mozzarella cheese, parmesan cheese, nutmeg, and chopped parsley. Mix with a spoon until combined.
Shape the mixture into balls, using the palms of your hands. Chill in the refrigerator for at least 15 minutes, or until ready to cook.
Beat two eggs and salt to taste. Dip each croquette in the beaten eggs and roll in the bread crumbs. To make a thicker, super crispy outer shell repeat this twice. Dip in egg and bread crumbs again.
In a small saucepan, heat one inch of vegetable oil over medium-high heat, until the oil is hot enough - you can test with a wooden spoon. The oil is ready when you see bubbles coming up around the spoon.
Fry the potato croquettes in batches until golden brown. Turn on all sides. It should take about two minutes per batch. Place on paper towel to drain. Serve with a marinara sauce!