Into a large sauté pan add 1/4 cup water and heat over medium heat. When simmering, add broccolini and cook until just softened. Transfer to serving bowl. Set aside.
Into a small bowl whisk together: soy sauce, vinegar, oil, red pepper flakes, sesame seeds and garlic. Pour into pan and heat over medium heat.
Add mushrooms and cook until tender. Pour over broccolini. Toss to combine.