Prepare the Beef: Start by slicing the flank steak thinly against the grain.
Marinate the Beef: In a bowl, mix together hoisin sauce, soy sauce, sesame oil, garlic, ginger, brown sugar, and black pepper. Add sliced beef and toss to coat. Let it sit for 10 minutes.
Cook the Noodles: Cook noodles according to package instructions, drain, and set aside.
Add the Beef: Push veggies aside, add marinated beef, and stir-fry until browned, about 3-5 minutes.
Combine the Noodles and Greens: Add cooked noodles, spinach, hoisin sauce, soy sauce, oyster sauce, rice vinegar, sriracha, garlic, ginger, and a splash of water. Toss to coat.
Thicken the Sauce: Mix cornstarch with water and add to the pan. Stir for another 2 minutes until glossy.
Serve: Remove from heat and serve hot, garnished with toppings.
Notes
Feel free to swap out veggies based on what you have. Marinate the beef for enhanced flavor and tenderness.