Rinse the sushi rice under cold water until the water runs clear. Drain well.
In a medium saucepan, combine the rinsed rice and 4 cups of water. Bring to a boil, then reduce the heat to low, cover, and simmer for about 15-20 minutes, or until the rice is cooked and the water is absorbed.
In a small saucepan, heat the rice vinegar, sugar, and salt over low heat, stirring until the sugar and salt dissolve. Remove from heat.
Transfer the cooked rice to a large mixing bowl. Pour the seasoned vinegar over the rice and gently fold it in to evenly distribute. Allow the rice to cool to room temperature.
Once the rice has cooled, add the chopped vegetables, soy sauce, sesame oil, and toasted sesame seeds. Mix well to combine.
Grease an 8x8-inch (20x20 cm) baking dish with cooking spray or oil.
Transfer the rice mixture into the prepared baking dish and press it down firmly to compact it.
Bake in a preheated oven at 350°F (175°C) for about 20-25 minutes or until the top is golden brown and crispy.
Remove from the oven and let it cool for a few minutes before cutting it into squares. Serve warm.