First, one caveat about the tofu. You can add texture to untextured soy bean curd by freezing it and then thawing it before preparation.
Begin by cooking the onion with the oil in a large saucepan until soft (the larger the pan, the better). Deglaze pan with 2 cups water. Add the remaining water, reseving the 1/2 cup for later. Simmer covered for 30 minutes. Add potatoes and cook until tender. In a blender or food processor puree the chilies, cilantro and 1/2 cup water. Add to the potatoes along with the zucchini, oregano, garlic, grated potato, salt and pepper. Cook 15 minutes. Serve in large bowls and top with sour cream.