In a large skillet, heat the vegetable oil over medium-high heat. Add the cubed tofu and fry until browned on all sides. Remove the tofu from the skillet and set aside.
In the same skillet, add the chopped onion, minced garlic, and grated ginger. Stir-fry until fragrant, about 1 minute.
Add the sliced bell peppers to the skillet and stir-fry until they are tender, about 5 minutes.
In a small bowl, whisk together the vegetable broth, soy sauce, hoisin sauce, rice vinegar, and brown sugar. Pour the mixture into the skillet and bring to a simmer. In a separate bowl, whisk together the cornstarch and water. Slowly pour the mixture into the skillet while stirring constantly, until the sauce thickens.
Add the fried tofu to the skillet and stir to coat it in the sauce. Simmer for an additional 5-10 minutes, until the tofu is heated through.
Season the dish with salt and pepper to taste. Garnish with chopped scallions before serving