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 stir-fry for about 3-4 minutes until browned. Remove the beef from the skillet and set aside.

In the same skillet, add the garlic and ginger, cooking for about 30 seconds until fragrant.

Add the bell pepper and snap peas, stir-frying for another 2-3 minutes until the vegetables are tender-crisp.

Return the beef to the skillet. Add the cooked udon noodles, soy sauce, oyster sauce, sesame oil, brown sugar, and red pepper flakes (if using).

Toss everything together and cook for an additional 2-3 minutes until heated through.

Serve hot, garnished with chopped green onions and sesame seeds.