Cook the udon noodles according to package instructions. Drain and set aside.
In a large skillet or wok, heat the vegetable oil over medium-high heat. Add the sliced beef and cook for about 3-4 minutes, or until browned. Remove the beef from the skillet and set aside.
In the same skillet, add the minced garlic and stir-fry for about 30 seconds until fragrant.
Add the broccoli, bell peppers, and carrot to the skillet. Stir-fry for about 4-5 minutes until the vegetables are tender-crisp.
Return the cooked beef to the skillet and add the cooked udon noodles.
Pour in the soy sauce, oyster sauce, mirin, and sesame oil. Toss everything together until well combined and heated through, about 2-3 minutes. Season with salt and pepper to taste.
Remove from heat and stir in the chopped green onions.
Serve hot, garnished with sesame seeds if desired.