Bring water to a boil. Cook the rice according to the instructions on the package. Drain and set aside.
Mince the onion. Wash the bell peppers. Cut them into quarters to remove the seeds, then into bite-sized pieces (see step-by-step pictures in post above).
Press excess moisture out of the tofu block (see notes). Cut it into small cubes. Place them in a bowl and add the cornstarch. Make sure the cubes are well coated.
Heat a large pan with vegetable oil. Make sure the pan is very hot before you add the tofu. Fry for 1 minute over high heat. Flip the tofu cubes and fry for another minute. Remove from the pan and set aside.
Add the onion and the bell peppers to the pan and sauté over medium heat for 10 minutes until the peppers are golden.
In the meantime, make the sweet and sour sauce. Add the pineapple juice, tomato concentrate, maple syrup, tamari, apple cider vinegar and a pinch of ginger powder (if using) to a bowl. Whisk until combined.
Add the tofu back to the pan with the sweet and sour sauce. Simmer for 5 minutes while stiring occasionally. The sauce will thicken.
Serve over rice and sprinkle with sesame seeds, if desired.