Prepare the beef by patting the flank steak dry with paper towels, then slicing it very thinly against the grain into bite-sized strips so it cooks quickly and stays tender.

Place the sliced beef in a bowl and sprinkle with half of the cornstarch, the fine sea salt, and black pepper; toss gently with clean hands or tongs until the beef is evenly coated and set aside to marinate briefly while you prepare the sauce.

In a separate small bowl, whisk together the gluten-free tamari, gluten-free oyster sauce, rice vinegar, water, and brown sugar until the sugar is mostly dissolved and the mixture looks smooth.

Add the remaining cornstarch to the sauce mixture and whisk again until no lumps remain; this will help thicken the sauce later in the pan.

Wash and cut the broccoli into small, even florets so they cook at the same rate, then mince the garlic and grate the fresh ginger, keeping them ready near the stove for quick cooking.

Heat one tablespoon of the neutral oil in a large skillet or wok over medium-high heat until it is shimmering but not smoking, indicating it is hot enough to sear the beef.

Add the coated beef to the hot pan in a single layer, working in batches if necessary to avoid crowding, and sear for 1 to 2 minutes per side until lightly browned but not fully cooked through; transfer the beef to a plate and repeat with any remaining beef, adding a little more oil if the pan looks dry.

In the same skillet or wok, add the remaining tablespoon of neutral oil if needed, then add the broccoli florets and stir-fry for 2 to 3 minutes until they brighten in color and start to become crisp-tender.

Pour in the gluten-free beef broth (or water) to the pan with the broccoli, quickly cover with a lid if you have one, and let the broccoli steam for another 2 minutes until just tender but still vibrant and slightly crisp.

Remove the lid, add the minced garlic and grated ginger to the broccoli, and stir-fry for about 30 seconds until fragrant, being careful not to let the garlic brown.

Return the seared beef and any accumulated juices from the plate back into the pan with the broccoli, stirring to combine everything evenly.

Give the prepared sauce a quick stir, then pour it over the beef and broccoli, tossing everything together so the sauce coats all the pieces.

Let the mixture simmer for 2 to 3 minutes, stirring occasionally, until the sauce thickens to a glossy consistency and the beef is just cooked through but still tender.

Turn off the heat and drizzle the toasted sesame oil over the stir-fry, gently folding it in to add aroma and a final layer of flavor.

Sprinkle the sliced green onions over the top as a fresh garnish, then serve the gluten-free beef and broccoli hot while the sauce is still silky and the broccoli is pleasantly crisp-tender.