Press Tofu: Cut tofu in half lengthwise, so you have two thinner rectangles, then wrap in clean kitchen towels. Place on a dinner plate or cutting board. Weigh tofu down with a heavy object like a cast iron skillet, or large pot for at least 20 minutes. Alternatively, use a tofu press.

Make Sauce: In a medium bowl, whisk together ketchup, vinegar, soy sauce, sugar, chili flakes, garlic and cornstarch-water mixture, until combined.

Coat Tofu: When you're ready, cube the tofu into 1/2-inch pieces and add to a large bowl with the cornstarch, garlic powder, salt and pepper. Toss to coat until all pieces are covered in cornstarch.

Cook Tofu: In a large non-stick skillet, heat 1 ½  tbsp oil over medium-high heat.  When the pan is hot, add the tofu (depending on the size of your pan, you may have to do this in batches).  Cook for 6-8 mins, flipping pieces occasionally, until golden-brown and crisp on all sides. Transfer tofu to a plate and set aside. 

Cook Veggies: To the same pan, add 2 tsp oil. Add peppers and onions.  Cook for 5-7 min, stirring occasionally, until tender-crisp.

Finish Stir Fry: Give the sauce another stir then pour over the veggies and cook for 1-2 minutes until it starts to thicken. Add tofu and bring to a simmer and cook for 1-2 min, stirring continuously, until sauce is thickened and the tofu is well coated. Remove from heat.

Serve: Serve sweet and sour tofu over your favorite brown or white rice. Sprinkle with green onions and toasted sesame seeds.