Drain the 14 oz. block organic extra firm tofu and place on a plate. Wrap paper or kitchen towel around the tofu and gently press to remove excess water. Cut the tofu into 2 cm cubes.
Place a large skillet over medium heat and add 1 tsp. sesame oil. Once hot, add the tofu cubes and let them cook for 3 to 4 minutes on a side, then do the same on a few more sides until most sides are lightly golden and crisp.
While the tofu is cooking, this is a good time to cook noodles or rice, if serving with the tofu.
Towards the end of the tofu cooking, drizzle in the 2 tsp. soy sauce and sprinkle with the ¼ tsp. garlic powder. Continue to stir fry another minute, careful to not break up the tofu. Transfer the tofu to a plate.
Add another 2 tsp. sesame oil to the pan along with the 1 small onion (finely chopped). Cook for 3 minutes. Add the 6 oz. sliced mushrooms. Cook for 3 to 4 minutes, then add the 1.5 bell peppers (diced) and continue to cook another 4 to 5 minutes until the peppers are crisp-tender.
While the vegetables are cooking, whisk together the sauce ingredients (1/4 cup + 2 tbsp. Sumo Citrus Mandarin juice, 1 tbsp. rice vinegar, 1 tbsp. sweet chili sauce, 4 tsp. chili garlic sauce, 1 tbsp. freshly grated ginger, 1 tbsp. agave, 1 tbsp. soy sauce, 1 tsp. garlic powder, 2 tsp. sesame oil, 1 tbsp. ketchup, and the cornstarch slurry (2 tsp. cornstarch + 2.5 tsp. water)).
When the peppers are crisp-tender, add the tofu back to the pan. Push everything to the side of the pan and add the sauce. Let the sauce heat up and bubble for around a minute, then stir together with the tofu and vegetables.
Serve topped with 1 tbsp. sesame seeds and ¼ cup sliced scallions.