Cook udon noodles according to package instructions (if you cannot find udon noodles, you can always use egg noodles or another wide noodle). Drain and set aside.
In small bowl, whisk soy sauce, rice vinegar, brown sugar, garlic, ginger & sesame oil.
Heat vegetable oil in large skillet over med-high heat. Cook beef and onion until beef is browned and no longer pink.
Stir in peppers and sauce and continue cooking until crisp tender (approx 3-4 mins more).
Combine beef mix with noodles until combined.
Serve.