Prepare the Rice: Follow the instructions on the package for 2 cups of rice. Typically, this involves rinsing the rice and allowing it to soak for 30 minutes before cooking. While that is cooking, in a small saucepan, heat the rice vinegar, sugar, and salt until dissolved. Gently fold this mixture into the cooked rice, then let it cool on the kitchen counter until it comes to room temperature.
Prepare The Tofu: While the rice is soaking and cooking, drain and press the tofu using a tofu press or wrapping the block in a clean kitchen towel and placing a heavy skillet on top. Remove as much moisture as you can for about 10-15 minutes. In a bowl, make the marinade by combining soy sauce, sesame oil, sriracha, thinly sliced nori sheets (or seaweed flakes), and green onions. Slice the tofu into rectangles and place them on a small baking sheet or container. If you are using an 8 x 8 baking dish, 8 slices of tofu will fit perfectly. Spread the soy sauce mixture over the tofu, marinating all the tofu. Refrigerate while you prepare the Japanese mayo.
Prepare the Japanese Mayo: Whisk together the vegan mayonnaise, rice vinegar, sugar, garlic powder, onion powder, soy sauce, and turmeric until the sugar is dissolved and the mixture is smooth and uniform. Add more turmeric if you want it to be more yellow and match traditional Japanese mayonnaise.
Assemble the Sushi Bake: Preheat your oven to 375°F (190°C). In an 8x8 single-serving or ceramic baking dish, brush the bottom and sides with toasted sesame oil. Dip your hands or spoon in cold water and gently spread half of the sushi rice evenly in the baking dish, creating the first layer of your sushi bake. Add a thin layer of shredded carrots. Place the tofu rectangles in one layer across the top of the rice and carrots, pouring out all of the marinade onto the top, including the scallions and nori. Then, evenly cover the entire dish with the remaining rice over the top. Finally, spread about ¼ cup of the Japanese mayo over the top.
Bake: Place the baking dish in the oven and bake for 15-20 minutes. During the last few minutes, turn the oven to broil to caramelize the top.
Serve: Once baked, let the sushi bake cool slightly. Garnish with thin slices of avocado, scallions, cucumber, and shredded carrot, then sprinkle sesame seeds on top for added crunch and a beautiful presentation. Drizzle the leftover mayo over the top using a squeeze bottle or spoon. If desired, serve with squares of nori to scoop some sushi bake onto the nori, creating a delicious, hand-held treat.