Slice the steak into thin strips.
If using dried udon noodles, cook them according to the package instructions, then drain and set aside. For pre-cooked udon, simply set aside.
In a bowl, mix soy sauce, oyster sauce, and sesame oil. Marinate the steak strips in half of this mixture for at least 15 minutes.
Heat your Blackstone griddle to medium-high heat.
Lightly oil the griddle surface.
Cook the marinated steak strips until they are seared and cooked to your preference. Remove and set aside.
Add a bit more oil if needed. Stir-fry the bell peppers, onions, and mushrooms until they are tender-crisp.
Add the udon noodles to the griddle. Pour in the remaining soy sauce mixture. Toss everything together and cook until the noodles are heated through.
Add the cooked steak back to the griddle with the noodles and vegetables.
Toss everything together, ensuring the ingredients are well combined and coated with the sauce.
Garnish the steak udon with chopped green onions, sesame seeds, and optional chili flakes.
Serve hot directly from the griddle.