Slice the steak into thin strips against the grain.
If using dried udon noodles, cook them to al dente per the package instructions, drain, and set aside. For pre-cooked udon, set aside.
In a bowl, whisk together the soy sauce, oyster sauce, and sesame oil. Marinate the steak strips in half of this mixture for at least 30 minutes; reserve the other half for the noodles.
Heat your Blackstone griddle to medium-high and lightly oil the surface.
Spread the marinated steak in a single layer and sear until cooked to your preference. Remove and set aside.
Add a little more oil, then stir-fry the bell peppers, onions, and mushrooms until tender-crisp. Add the garlic and ginger and stir for about 30 seconds, until fragrant.
Add the udon noodles and the reserved sauce. Toss and cook until the noodles are heated through and lightly charred.
Return the steak to the griddle with the noodles and vegetables. Toss everything together until well combined and coated in sauce.
Garnish with green onions, sesame seeds, and chili flakes. Serve hot off the griddle.