In a small bowl, whisk together soy sauce, rice vinegar, vegan honey or maple syrup, sesame oil, and cornstarch until smooth. Set aside.
Slice all vegetables into bite-sized pieces.
Heat a wok or large skillet over medium-high heat and add sesame oil.
Once the oil is hot, add broccolini, bell peppers, and mushrooms. Stir-fry for about 7 minutes until vegetables are tender-crisp.
Pour the sauce over the vegetables and cook for 2–3 minutes, stirring constantly, until the sauce thickens and coats the vegetables evenly.
Remove from heat and garnish with sliced green onions and sesame seeds. Serve hot.