Step 1: Place diced potatoes in a medium pot and cover with cold salted water by 1 inch. Bring to a boil, reduce heat, and simmer for 12–15 minutes until fork-tender.
Step 2: Drain potatoes well and let them steam dry for 2–3 minutes. Transfer to a large bowl and mash until smooth (or use a ricer).
Step 3: While still warm, add butter, shredded cheddar, Dijon mustard, chopped scallions, salt, and pepper. Mix until well combined and cheese is melted.
Step 4: Spread the potato mixture on a baking sheet and chill in the fridge for 15–20 minutes until firm.
Step 5: Scoop tablespoon-sized portions and roll into balls (about 1–1.25 inches in diameter). Place on a lined tray.
Step 6: Set up a coating station: one bowl with flour, one with beaten eggs, and one with panko.
Step 7: Roll each potato ball in flour, dip in egg, then roll in panko, pressing gently to adhere.
Step 8: Heat 1–1.5 inches of oil in a deep skillet to 350°F. Fry bites in batches for 2–3 minutes, turning, until golden brown and crispy. Transfer to a rack to drain.