Add potatoes to a large pot of boiling water and cook until fork tender, about 20 minutes. Drain and add back to the pot, then mash with salt, pepper, butter and milk. Stir in parmesan and onions, then place the mixture in the fridge to cool for 15-20 minutes.
Preheat the oven to 400 degrees.
Remove potatoes from fridge and roll into 1-inch balls. Dip in beaten egg, then cover in bread crumbs. Place on a baking sheet 1-2 inches apart, and hit each with a quick spritz of olive oil or cooking spray. Bake for 12-15 minutes, or until outside is crispy.