Combine rice vinegar, soy sauce and sugar in a medium bowl. Add steak and toss to coat. Marinate 30 minutes or up to 24 hours.

Meanwhile, whisk together chicken stock, oyster sauce, soy sauce, cornstarch and sugar. Set aside.

Heat 2 tablespoons oil in a large skillet or wok over medium-high heat. Add garlic, chile, broccoli and onion and cook, stirring often, until broccoli turns bright green. Transfer to a large bowl and add remaining tablespoon oil to skillet. When shimmering hot, shake excess marinade off of beef and add it to pan in a single layer. Brown well on both sides. Return broccoli mixture to skillet, add sauce, and cook, tossing frequently, until sauce thickens and broccoli is crisp-tender, about 2 minutes.