In a bowl, toss the sliced steak with 1 tbsp tamari and 1 tsp cornstarch. Let sit for 10 minutes while you prep the vegetables.
Whisk together the remaining tamari, beef broth, brown sugar, rice vinegar, sesame oil, and remaining cornstarch in a small bowl. Set aside.
Heat 2 tbsp neutral oil in a large skillet or wok over high heat until shimmering. Add the beef in a single layer and sear for 1-2 minutes per side until browned. Transfer to a plate.
Add the remaining 1 tbsp oil to the pan along with the broccoli and onion. Stir-fry for 3-4 minutes until the broccoli is bright green and crisp-tender.
Add the garlic, ginger, and red pepper flakes and stir-fry for 30 seconds until fragrant. Return the beef to the pan, pour in the sauce, and toss for 1-2 minutes until the sauce thickens and coats everything glossy.
Garnish with sliced scallions and serve immediately over steamed rice.
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