Combine the thinly sliced flank steak and the listed marinade ingredients in a large mixing bowl. Toss to coat well, then set aside to marinate for 15 minutes.
In a small bowl, whisk together all of the listed noodle sauce ingredients until combined and set aside.
Place the noodles into a large bowl and pour boiling water over them. Let them sit for about 30 seconds just until they loosen up, then drain immediately and set aside.
Bring a large pan of water to a rolling boil over high heat. Add the broccoli florets and cook for 1 to 2 minutes until they are vibrant green and crisp-tender, then drain well, shaking off any excess water.
Heat vegetable oil in a large pan over medium-high heat. Arrange the marinated beef in a single layer and cook until it's mostly brown and slightly pink in the center.
Toss the onions into the pan and stir-fry until translucent. Add the loosened noodles and the prepared noodle sauce, tossing continuously until everything is evenly coated. Fold in the blanched broccoli florets, toss until combined and heated through, then remove from the heat and serve immediately.