First, prepare the tofu by pressing it for at least 30 minutes to remove excess moisture. Once pressed, cut it into bite-sized cubes.
In a bowl, toss the tofu cubes with cornstarch until evenly coated. This will help them crisp up during cooking.
Heat vegetable oil in a large non-stick skillet over medium-high heat. Once hot, add the tofu cubes and cook until golden brown on all sides, about 8-10 minutes. Remove the tofu from the skillet and set aside.
In the same skillet, add the sliced bell peppers and snap peas. Stir-fry for about 3-4 minutes until they are vibrant and slightly tender.
While the vegetables are cooking, in a bowl, whisk together soy sauce, rice vinegar, brown sugar, ginger, garlic, and sesame oil until combined.
Return the tofu to the skillet with the stir-fried vegetables and pour the sweet and sour sauce over the top. Gently toss everything together to ensure the tofu and vegetables are coated with the sauce. Cook for an additional 2-3 minutes until heated through.
Add the pineapple chunks in the last minute of cooking to warm them through.
Serve the tofu mixture over cooked rice or quinoa. Garnish with chopped green onions and sesame seeds.