In a mixing bowl, whisk together the cornstarch, soy sauce, mirin, and vegetable broth. Set aside.
Heat the oil in a large skillet over medium heat. Add the onion, garlic, and red bell pepper. Cook for 5 minutes or until the vegetables are tender.
Add the shiitake mushrooms and tofu to the skillet. Cook for another 5 minutes or until the mushrooms are tender and the tofu is lightly browned.
Pour the sauce mixture over the vegetables and tofu in the skillet. Bring to a simmer and cook for 5-10 minutes or until the sauce has thickened.
Add the nutritional yeast to the skillet and stir until well combined.
Divide the cooked rice among four bowls. Spoon the tofu and vegetable mixture over the rice.
Garnish with chopped green onions and serve hot.