Get the jasmine rice cooking first — it runs in parallel and is the 30-minute enabler. Slice the flank steak thinly (about 1/8 inch) against the grain into 2-inch strips. Tip: if you have 10 spare minutes, slide the steak into the freezer first — partially frozen meat slices paper-thin.

Toss the sliced beef with the cornstarch, salt, and pepper until every strip is lightly dusted. Heat 1 tablespoon oil in a large skillet over high heat until shimmering. Sear the beef in a single layer — work in two batches — 1 to 2 minutes per side, until browned and caramelized at the edges. Transfer to a plate. Crowded beef steams; spaced-out beef crisps.

Add the remaining 1 tablespoon oil to the same skillet. Add the broccoli and 1/4 cup water, cover immediately, and steam 2 minutes. Uncover and cook 2–3 minutes more, stirring occasionally, until the water evaporates and the florets pick up golden char spots.

In a small bowl, whisk together the soy sauce, brown sugar, ginger, garlic, cornstarch slurry, and red pepper flakes if using. It will look thin — the cornstarch fixes that in the pan.

Return the beef to the skillet with the broccoli. Pour the sauce over and toss 2–3 minutes, until it bubbles, thickens to a glossy glaze, and coats everything. The beef should register at least 145°F (it will, after the sear plus this finish).

Spoon over hot jasmine rice and finish with sliced green onions and sesame seeds. Drizzle any extra glaze from the pan over the top.