Bring a large pot of water to a boil over medium-high heat. Add chopped potatoes. Cook for 15 minutes until fork-tender. Drain well. Transfer to a large mixing bowl.
Heat 1 tablespoon (15 ml) vegetable oil in a pan over medium heat. Sauté onions until softened and translucent. Add to the potatoes and season with salt, garlic powder, black pepper, and curry powder. Chill in the fridge for 5 minutes before mashing.
Mash potatoes and onions just until combined, leaving small crumbs - do not over-mash. Divide into 8 equal portions.
Form each portion into oval-shaped balls and chill for 15 minutes. Do not skip this step, or they may fall apart during coating.
Set up three shallow plates: cornstarch, beaten egg, and panko breadcrumbs. Coat each ball in cornstarch, then egg, then breadcrumbs until fully covered.
Heat 2 inches of vegetable oil to 325-350ºF (or insert a wooden chopstick into the oil and look for rapid bubbles). Fry in small batches for 3-4 minutes until golden and crispy, flipping halfway. Drain on a wire rack.
In a small bowl, combine the listed ingredients for the tonkatsu sauce until well combined and serve with korokke.