In a large nonstick skillet, saute onion until very soft. Add garlic and continue to saute for 2-3 minutes. Add mushrooms and zucchini and continue to cook until all veggies are soft. Add cumin, chili powder, salt, and pepper.
Stuff peppers until they are half full with cooked veggie mixture. Add a layer of shredded cheese. Fill with more veggie mixture and top with cheese and place in a baking dish. Repeat until all peppers are filled. Cook in the oven until they are heated through and the cheese is melted. Top with a drizzle of the warmed enchilada sauce and a dollop of sour cream and serve.