Cut the tofu into bite-sized cubes and pat them dry with a paper towel. In a bowl, toss the tofu with soy sauce and cornstarch to coat evenly.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the tofu cubes and cook for 4–5 minutes on each side until golden and crispy. Remove from the pan and set aside.
Add the remaining tablespoon of oil to the same pan. Toss in the bell pepper, broccoli, carrot, zucchini, and mushrooms. Stir-fry for 3–4 minutes until the vegetables become slightly tender but still crisp.
Stir in the minced garlic and grated ginger. Cook for another 30 seconds until fragrant.
In a small bowl, whisk together soy sauce, hoisin sauce, sesame oil, rice vinegar, maple syrup (or honey), red pepper flakes (if using), and water.
Return the crispy tofu to the pan. Pour the sauce over the vegetables and tofu, stirring to coat everything evenly. Let it cook for another 2 minutes until the sauce thickens slightly.
Sprinkle sesame seeds and chopped green onions on top. Serve immediately over rice or noodles.
