In a bowl, mix together soy sauce, cornstarch, and sesame oil until well combined.
Add tofu cubes to the bowl and toss to coat them in the sauce.
Heat vegetable oil in a large skillet or wok over medium-high heat.
Add garlic and ginger to the skillet and cook for 1 minute.
Add sliced bell peppers, carrot, and broccoli to the skillet and stir fry for 5-7 minutes, until the vegetables are tender-crisp.
Push the vegetables to one side of the skillet and add the marinated tofu cubes to the other side.
Cook the tofu for 3-4 minutes on each side, until golden brown.
Season the stir fry with salt and black pepper, to taste.
Garnish with chopped green onions and sesame seeds.
Serve the tofu stir fry hot over steamed rice or noodles.