Using the same wok over medium-high heat, add in the remaining 1 tbsp of vegetable oil. Add in the remaining half of the sliced onions to get rid of the brown bits stuck on the bottom of the wok. Add in the green bell pepper, udon, Swiss sauce mixture, and stir fry for around 2-3 minutes. Add back in the beef onion mixture, followed by the green onion, black pepper, and mix until everything is combined. Serve and enjoy!