Boil chunked potatoes with 1 tablespoon salt in a large pot until fork-tender, about 15-20 minutes. Drain and mash until smooth. Set aside to cool slightly.
In a large skillet, sauté chopped onions until translucent, about 5 minutes. Add sweet corn and cook for another 2-3 minutes.
In a small bowl, dissolve Japanese curry cubes in 1/4 cup hot water, stirring until smooth to create curry sauce.
Mix the curry sauce, sautéed onions, corn, and mashed potatoes together. Season with salt and pepper. Allow mixture to cool until easy to handle.
Shape the potato mixture into oval portions, placing a cube of mozzarella in the center of each and sealing completely.
Prepare coating station: whisk together flour, cold water, and eggs in one bowl. Place panko breadcrumbs in another bowl.
Dip each croquette in the batter, guaranteeing complete coverage, then roll in panko breadcrumbs.
Heat oil to 350°F (180°C) in a deep fryer or heavy pot. Fry croquettes for 3-4 minutes per side until golden brown and crispy.
Remove and drain on paper towels. Serve immediately while hot and crispy.