1Prepare all vegetables: slice onions, julienne carrots, slice mushrooms, and cut bell pepper into strips. Thinly slice beef if not already done.
2Cook frozen udon noodles in boiling water for 2-3 minutes until just tender. Rinse and drain under cold water. Drizzle with 1 tablespoon sesame oil and mix to prevent sticking.
3Heat 1 tablespoon olive oil in a large pan or wok over medium-high heat. Add minced garlic and sauté until fragrant.
4Add thinly sliced beef in an even layer. Season with salt, black pepper, 2 teaspoons soy sauce, and 2 teaspoons dark soy sauce. Cook without stirring for 2 minutes, then stir and cook for another 1-2 minutes until browned. Remove beef from pan and set aside.
5In the same pan, add another tablespoon olive oil and a little garlic. Add mushrooms and cook for 1 minute.
6Add onions, carrots, and bell pepper. Stir-fry for 3-5 minutes until vegetables are just tender.
7Add cooked udon noodles to the pan and mix with the vegetables.
8Return cooked beef to the pan and mix well.
9In a bowl, combine 1 tablespoon brown sugar, 3 teaspoons dark soy sauce, 3 teaspoons low sodium soy sauce, 4 teaspoons rice vinegar, 2 tablespoons oyster sauce, 1 tablespoon sesame oil, salt, and black pepper. Pour sauce over the noodles and stir everything together until well combined.
10Turn off heat and finish with chopped scallion. Serve hot.