Prep all vegetables: slice onions, julienne carrots, slice mushrooms, and cut bell pepper into strips. Thinly slice beef if needed.
Cook frozen udon noodles in boiling water for a couple of minutes. Rinse and drain well under cold water. Drizzle with sesame oil and mix to prevent sticking.
Heat olive oil in a pan and sauté garlic until fragrant. Add thinly sliced beef in an even layer and cook untouched for a couple of minutes until it starts to brown.
Season beef with salt, black pepper, 2 tsp soy sauce, and 2 tsp dark soy sauce. Mix and cook for another 1-2 minutes. Remove beef from pan.
In the same pan, heat oil and add mushrooms. Cook for approximately 1 minute.
Add onions, carrots, and bell pepper. Cook for 3-5 minutes.
Add cooked udon noodles and mix.
Add cooked beef and mix.
Prepare sauce by combining 1 TBSP brown sugar, 3 tsp dark soy sauce, 3 tsp low sodium soy sauce, 4 tsp rice vinegar, 2 TBSP oyster sauce, 1 TBSP sesame oil, salt and black pepper. Pour sauce into the pan and mix until everything is well combined.
Turn off heat and finish with chopped scallion.
