In a skillet over medium heat brown the bratwurst while breaking it up with a wooden spatula or spoon.* Add the diced onion, mix it in and let soften. Once the bratwurst is fully cooked let the mixture cool down.
Over medium-high in a deep frying pan heat enough cooking oil to fully submerge the balls to 350 F.
Bread the chilled sauerkraut balls (coat with flour, then egg wash, then roll in bread crumbs).
Fry a few at a time until golden brown and drain on paper towels. Serve immediately.