Cut onion, green onion, zucchini, and shiitake mushrooms into bite-sized pieces.
Add cooking oil to a pan and sauté onion and green onion.
Add zucchini and mushrooms and stir-fry together.
Season with chili powder, minced garlic, and soy sauce.
Pour in water or vegetable broth and bring to a boil.
Add soft tofu and gently stir while simmering.
Season to taste with salt and pepper.
Top with chopped scallion to finish.