Pat the tofu dry with paper towels, then cut it into bite-sized cubes. Toss the tofu with soy sauce, black pepper, and cornstarch until evenly coated.
Heat the oil in a large skillet or wok over medium-high heat. Arrange the tofu in a single layer and cook for 8 to 10 minutes, turning occasionally, until golden and crisp on all sides. Transfer the tofu to a plate.
While the tofu cooks, whisk together the soy sauce, vegetable broth, rice vinegar, maple syrup, sesame oil, cornstarch, and sriracha until smooth.
Using the same skillet, add another drizzle of oil if needed. Cook the broccoli, carrots, and bell peppers for about 4 minutes, stirring frequently. Add the snap peas and continue cooking for another 2 minutes until the vegetables are crisp-tender. Stir in the garlic, ginger, and most of the green onions. Cook for about 30 seconds until fragrant.
Return the crispy tofu to the skillet. Pour in the prepared sauce and gently toss everything together. Continue cooking for 2 to 3 minutes until the sauce thickens and evenly coats the tofu and vegetables.
Remove from the heat and garnish with sesame seeds and the remaining green onions. Serve immediately over steamed rice or noodles.