In a large saucepan, combine the water, onion, salt, and cayenne. Bring to a boil, reduce heat to a simmer, cover, and cook for 5 minutes, or until the onion is soft.
Add the bell pepper, tomatillos, and chilies. Cover and simmer until the tomatillos are soft and have released their juices, about 10 to 15 minutes. Using an (immersion) blender or food processor, puree the mixture. Taste and adjust the seasoning. Add a little sugar if the sauce is too acidic.
Filling: In a large skillet over medium-high heat, heat the oil and saute the red onion, salt and cayenne for 8 minutes, or until the onion is wilted. Add the garlic, bell pepper, zucchini, and cumin. Cook for 5 minutes. Add the mushrooms, corn, and beans. Cook for another 5 minutes until the mushrooms are soft and the corn is crisp tender. Stir in cilantro. Remove from heat.
Putting it all together:
Preheat the oven to 400 degrees.
Ladle 1 cup tomatillo sauce into the bottom of a 9x9 baking pan.
Heat a tortilla on a comal or frying pan over medium-high heat until pliable. Lay the tortilla on a clean surface and spread 1 tablespoon sour cream down the center of the tortilla. Spread 3 tablespoons of the filling over the sour cream and then sprinkle with 2 tablespoons grated cheese. Roll the tortilla up and place seem side down in the baking pan. Repeat with all remaining tortillas. Ladle the sauce on top, sprinkle with cheese.
Cover with foil and bake 20 minutes, or until the sauce is bubbling. Remove foil and bake 5-7 minutes more until cheese is browned. Sprinkle with cilantro and serve.