How to Make It
In a saucepan, cover the potatoes with cold water. Bring to a boil, add a generous pinch of salt and simmer over moderate heat until tender, 20 minutes. Drain and let cool. Pass the potatoes through a ricer into a large bowl. Stir in the mozzarella and parsley and season with salt and pepper. Shape the mixture into 8 oval croquettes and transfer to a plate. Cover and refrigerate until firm, 30 minutes.
Meanwhile, in a saucepan, heat the olive oil. Add the tomatoes and cook over moderate heat, stirring, until softened and saucy, 8 minutes. Stir in the basil and season with salt and pepper. Transfer to a bowl.
Spread the bread crumbs in a shallow bowl. In another shallow bowl, beat the eggs with the mustard, garlic and a pinch each of salt and pepper. Dredge the croquettes in the bread crumbs, tapping off the excess. Dip the croquettes in the beaten egg mixture to coat, then dredge again in the bread crumbs, pressing lightly to help the crumbs adhere.
In a large saucepan, heat 1 inch of vegetable oil to 350°. Working in 2 batches, fry the croquettes, turning, until golden and crisp, about 3 minutes per batch. Transfer to paper towels to drain. Serve with the tomato sauce.