Open the package of tofu and drain it completely. Use a tofu press if you have one, otherwise, layer the tofu with clean towels and place a heavy object on it to press out excess liquid. Let it press for at least 15-20 minutes.
Preheat the oven to 400 degrees Fahrenheit and prepare a baking sheet lined with parchment paper. Cut the pressed tofu into bite-sized cubes, and arrange them on the baking sheet. Don't crowd the pan - use a second baking sheet if needed.
Bake for approximately 35-40 minutes, flipping the pieces over after 20 minutes, or until lightly browned and crisp on the outside. The tofu will continue to get crispier as it cools.
Meanwhile, prepare the sweet and sour sauce base:
In a wok or large pan, warm the oil over medium-high heat. Once the oil is hot, add the bell pepper and continue to cook, stirring frequently, until the bell pepper is juicy and softened somewhat, 3-5 minutes. Add the garlic and white pepper (if using), stir, and cook for just 30 seconds more.
Add the pineapple juice and stir, scraping around the bottom of the pan. Bring the mixture to a boil, then reduce to a simmer, and cook until the pineapple juice has reduced somewhat.
Add the rest of the sauce ingredients EXCEPT for the cornstarch, and stir to combine. Taste for seasoning, and add more soy sauce, vinegar, or sriracha if desired.
Whisk the cornstarch together with 2 tablespoons of cold water, then add that mixture to the pan and stir well. Continue to cook over medium-low heat until the sauce has become glossy and thickened slightly. If the tofu isn't done baking, you can turn off the heat and wait at this point.
After taking the tofu out of the oven, add it to the pan with the sauce, and stir together to coat the tofu. Serve hot, garnished with scallions and sesame seeds if desired. I like mine with a side of rice.
Notes
CRISPY BREADED SWEET AND SOUR TOFU: In a small bowl, mix together 1/4 cup cornmeal, 1/4 cup flour, 1/4 tsp salt, 1/4 tsp garlic powder, and a shake of black pepper. Whisk together all of the sweet and sour sauce ingredients except for the cornstarch. Dip your tofu cubes in the sauce mixture, let the excess drip off, and then toss them in the flour mixture. Spray the baking sheet with oil and then bake the breaded tofu cubes on the baking sheet, flipping over after 20 minutes, until crispy and lightly browned. Then, proceed with the recipe as otherwise instructed.