In a small bowl, whisk together the soy sauce, hoisin sauce, brown sugar, cornstarch, sesame oil, garlic, and ginger. Set aside.
Heat a large skillet or wok over high heat. Add the flank steak and cook until browned and cooked through, about 2-3 minutes per side. Remove from heat.
Pour the sauce over the cooked steak and toss to coat. Let the steak marinade in the sauce for a few minutes.
In a large salad bowl, combine the mixed greens, cherry tomatoes, cucumber, carrots, red onion, and cilantro. Toss to combine.
Transfer the salad to serving plates and top with the marinated steak. Serve immediately.