Heat 1 tablespoon of oil in a large skillet or wok over high heat. Once hot, add the sliced mushrooms, snap peas, shredded carrots, and sliced red pepper. Stir occasionally and cook until vegetables are crisp-tender, about 5 minutes. Transfer the vegetables to a plate and set aside.
Slice the flank steak against the grain into 1/4-inch thick strips, then cut into 2-inch pieces. Toss the steak pieces with corn starch in a bowl.
In the same skillet, add 1 tablespoon of oil. Once hot, add half the beef, spreading it out in a single layer. Cook without stirring for 1 minute until well seared. Stir and cook for an additional 40 seconds until lightly browned but still slightly pink. Transfer the beef to a plate. Repeat with the remaining beef and oil, then transfer to the plate. Do not wash the pan.
In the same skillet, add beef broth, soy sauce, sesame oil, garlic, ginger, brown sugar, and red pepper flakes. Stir, scraping up all the browned bits from the bottom of the pan. Bring the mixture to a boil and let it reduce by about 1/3, approximately 5–8 minutes. Ensure you leave enough sauce to coat the noodles and veggies. If sauce hasn’t thickened nicely, see note 3.
Discard the seasoning packets that come with the ramen noodles. While the sauce is reducing, bring a large pot of water to a boil. Cook the ramen noodles for exactly 2 minutes. Drain and rinse with cold water for 10 seconds. Shake off any excess water and set aside.
Add the cooked vegetables, cooked beef, and drained ramen noodles to the sauce. Use tongs to toss until everything is well coated. If desired, sprinkle green onions and sesame seeds on top. Enjoy immediately.
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Notes
Note 1: Optional marinade: Mix 1/4 cup soy sauce, 2 tablespoons hoisin sauce, and 1 tablespoon minced garlic. Knead the marinade into the cut steak in a zip-top bag and let it sit at room temperature for 20–30 minutes, or refrigerate for up to 8 hours.Note 2: If you’re unsure about the sweetness, start with less brown sugar and adjust to taste. You can also add more red pepper flakes (up to 1/2 teaspoon more) to balance the sweetness.Note 3: If the sauce reduces by 1/3 but isn’t thick enough, mix 1 teaspoon of the sauce with 1 teaspoon of corn starch in a separate small bowl. Whisk them together, then add the mixture back to the sauce and stir until it thickens.Storage: This dish is best fresh. Store leftovers in the fridge for up to 2 days. Reheat gently on stovetop or in the microwave. Note the noodles soften as they sit.