First, you'll want to slice your flank steak thoughtfully. Slicing against the grain means each bite will be tender. Once you’ve sliced the meat, sprinkle it with 1 tablespoon of cornstarch. Toss the beef until it’s lightly coated. This step is crucial as it helps to create a velvety sauce that clings beautifully to the meat.

In a mixing bowl, whisk together the soy sauce, brown sugar, minced garlic, grated ginger, and rice vinegar. This combination will transform into a flavor-packed marinade that permeates the meat. Allow the beef to marinate for at least 15 minutes. If you have the time, letting it sit longer will deepen the flavors further.

In a skillet over medium-high heat, add 1 tablespoon of vegetable oil. Once it's hot, toss in the sliced onions. Sauté until they become tender and translucent, about 3 minutes. Next, remove the onions and add the marinated beef to the pan. Stir-fry it for about 3-5 minutes until the beef is browned. Be careful not to overcrowd the pan. You want that gorgeous sear!

With the beef cooked through, remove it from the skillet. Set it aside and keep it warm. In the same skillet, add another tablespoon of vegetable oil and toss in the broccoli florets. Stir fry for about 5 minutes until the broccoli is bright green and tender-crisp.

Now, return the beef to the skillet and pour the remaining marinade over everything. Give it a generous toss, coating both the broccoli and the beef. Allow everything to simmer for another minute. This step ensures the flavors meld beautifully. Don’t forget to drizzle the sesame oil over the top right at the end for that rich, aromatic finish.

Serve your beef and broccoli hot over a bed of rice, quinoa, or cauliflower rice. If you're feeling fancy, a sprinkle of sesame seeds or chopped green onions can elevate the dish’s presentation. Enjoy every savory bite!