Whisk together 2 tablespoons of the soy sauce and the cornstarch. Pour over sliced beef and toss to coat. Set aside.
Prepare and assemble the rest of your ingredients before you begin cooking.
Set a medium pot of water on to boil for the ramen.
While the ramen water heats up, in a large skillet with a lid, heat vegetable oil over medium-high heat. Add the sirloin and let it cook undisturbed until the first side is well browned, about 1 to 2 minutes. Stir and continue to cook until the steak reaches your desired level of doneness, about 1 to 2 minutes (this is all the cooking it will get).
Transfer meat and any drippings to a bowl and set aside.
Allow the pan to cool for just a minute and then add the sesame oil. Heat over medium heat and add garlic and ginger. Cook until the garlic is just beginning to turn golden and aromatic. Add red pepper flakes and cook for 30 more seconds.
Add the broth, remaining ¼ cup of soy sauce, brown sugar, and vinegar, and use a spatula to scrape any cooked-on bits from the bottom of the pan. Bring to a simmer and cook until the sauce is reduced by about a third, around 3 to 6 minutes.
Add broccoli, place lid on the skillet, and cook for two minutes. Give the broccoli a stir, replace the lid, and continue to cook until it reaches your preferred level of doneness, 1 to 3 minutes more.
While the broccoli cooks, add ramen to the boiling water and cook according to package instructions.
Stir cooked beef and any juices in the bowl into the broccoli and sauce. Add cooked and drained ramen noodles and toss until the noodles are well-coated with sauce.
Serve topped with sliced green onions if desired, and enjoy!