6 medium russet (baking) potatoes (2 1/2 pounds), peeled and quartered
1/2 lb mozzarella, chopped
1/4 Cup grated Parmesan
1 Tbsp Onion Salt
1/4 Cup chopped parsley
3 large eggs
1 Cup fine dry bread crumbs
About 2-1/2 Cups Extra Virgin Olive Oil
Generously cover potatoes with salted cold water (2 teaspoon for 4 quarts) in a large pot, bring water to a boil, then reduce to a simmer and cook until tender, about 15 minutes. Drain potatoes and chill in a large bowl until fully cooled, about 45 minutes.
Mash potatoes, then stir in cheeses, onion salt, parsley, 1/2 teaspoon black pepper, and kosher salt to taste. Stir in 1 egg.
Form potato mixture into 24 oblong croquettes (about a scant 1/4 cup for each). Lightly beat remaining 2 eggs in a shallow bowl and put bread crumbs in another shallow bowl. Dip 1 croquette into egg, letting excess drip off, then roll eggy potato croquette in bread crumbs to coat. Transfer to a parchment-paper-lined baking sheet. Repeat with remaining croquettes.
Heat 1/2 inch oil in a 12-inch heavy skillet over medium-high heat until surface of the oil shimmers. Fry croquettes in batches, turning occasionally, until golden brown, 4 to 5 minutes per batch. Transfer to paper towels to drain. Serve immediately.