Combine brown sugar, fish sauce and beef stock in a pot, boil until sugar dissolves, place to one side. Season beef, cook to seal in hot wok with oil. Remove and drain. Sweat onion, garlic, carrot in remaining oil, add chili and pepper, salt to taste. Return beef to wok and add sauce. Finish with chiseled spring onion. Cook Udon according to packet directions.
Serve in deep plates on top of the noodles