Step 1: Marinate the Beef. In a medium bowl, combine the thinly sliced beef with 1 tablespoon of soy sauce and 1 teaspoon of cornstarch. Mix well and let it marinate for at least 15 minutes while you prepare other ingredients.
Step 2: Prepare the Sauce. In a small bowl, whisk together 1/4 cup soy sauce, oyster sauce, mirin, rice vinegar, sesame oil, 1 teaspoon cornstarch, and water/beef broth until smooth. Set aside.
Step 3: Cook the Noodles. Cook the lo mein noodles or spaghetti according to package directions until al dente. Drain well. In the same pot, melt butter with 1 tablespoon of neutral oil. Add 3 cloves of minced garlic and sauté for 1 minute until fragrant. Add the cooked noodles, tossing to coat. Season with salt and pepper. Set aside.
Step 4: Stir-fry the Beef. Heat 1 tablespoon of neutral oil in a large wok or skillet over high heat until shimmering. Add the marinated beef in a single layer (cook in batches if necessary to avoid overcrowding). Cook for 1-2 minutes per side until nicely browned, but still slightly pink in the center. Remove the beef from the wok and set aside.
Step 5: Sauté Mushrooms & Aromatics. Add another 1 tablespoon of neutral oil to the same wok. Add the sliced mushrooms and cook for 5-7 minutes, stirring occasionally, until deeply golden brown and softened. Push mushrooms to one side. Add the remaining 4 cloves of minced garlic, grated ginger, and the white parts of the green onions to the empty side of the wok. Sauté for 1 minute until fragrant.
Step 6: Combine and Finish. Return the cooked beef to the wok with the mushrooms and aromatics. Give the prepared sauce a quick whisk (as cornstarch can settle) and pour it over the beef and vegetables. Bring the sauce to a simmer, stirring constantly, until it thickens slightly and coats everything beautifully (about 1-2 minutes).
Step 7: Serve. Immediately add the garlic noodles to the wok and toss to combine, ensuring the noodles are coated with the umami-rich stir-fry sauce. Garnish with the green parts of the sliced green onions and sesame seeds, if desired. Serve hot and enjoy this amazing umami experience!