Chop broccoli into even, bite-size pieces. Slice the flank steak against the grain into 1/4-inch-thick strips, then cut into 2-inch pieces. Toss the steak pieces with cornstarch in a bowl.
Heat 1 tablespoon of vegetable oil in a large pan or wok over high heat. Once oil is hot, add half the beef, spacing it out in a single layer. Cook without stirring for about 1 minute, until well seared. Stir and cook until lightly browned but still slightly pink, about 40 seconds. Transfer to a plate. Repeat with the remaining beef and oil, then transfer to the plate.
In the same pan, add the beef broth, brown sugar, soy sauce, sesame oil, oyster sauce, garlic, and ginger. Stir and scrape up any browned bits from the bottom of the pan. Cook over high heat, uncovered, until the sauce boils and thickens slightly; this takes about 5–8 minutes. Make sure there’s enough sauce to coat the noodles and broccoli. See note 3.
Once the sauce is reduced, evenly place the broccoli on top without stirring it in. Cover the skillet with a lid, reduce heat to low, and steam the broccoli about 2–4 minutes.
Discard seasoning packets that come with the ramen noodles. While the broccoli steams, bring a large pot of water to a boil and 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.
Remove the lid from the pan, add the drained ramen and cooked beef to the sauce and broccoli. Gently toss everything to coat well. Garnish with sesame seeds, green onions, and red pepper flakes if desired. Serve immediately.
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Notes
Note 1: Follow these instructions to create an optional marinade for the beef. In a bowl, whisk together 1/4 cup vegetable oil, 1 teaspoon minced garlic, 2 tablespoons red wine vinegar, 1/4 cup honey, 1/4 cup soy sauce, 1/4 teaspoon pepper, 1/2 teaspoon salt, and 1 tablespoon coarsely chopped Italian parsley. Pour the mixture over the flank steak in a zip-top bag, seal, and refrigerate for at least 1 hour, up to 8 hours.Note 2: Some people find 1/3 cup of brown sugar too sweet. If you're unsure, start with less, and adjust to taste. Adding red pepper flakes can also help balance the sweetness.
Note 3: If the sauce reduces by 1/3 but isn't thick enough, take 1 teaspoon of the sauce and mix it with 1 teaspoon of cornstarch in a separate small bowl. Whisk them together, then add the mixture back to the sauce. Stir until it thickens.Storage: This Beef and Broccoli Ramen is best enjoyed fresh as the noodles get softer over time. Any leftovers can be stored in an airtight container in the fridge for up to 2 days. Reheat on the stove or in the microwave.