sharp cheddar cheese and/or Monterey Jack, grated and packed
Preheat oven to 350°F. Heat olive oil in heavy large skillet over medium-high heat. Add zucchini and onion and sauté until vegetables are just tender, about 4 minutes. Mix in corn, beans, and drained tomato with chilies; season filling with salt and pepper. [Substitute any vegetables you may have on hand e.g., sweet potato, cauliflower, other types of squash all work well.]
Meanwhile, melt butter in heavy medium saucepan over medium-high heat. Whisk in chili powder, flour and cumin; stir 30 seconds. Gradually whisk in about one cup water. Cook until sauce thickens and is bubbling, whisking occasionally, about 5 minutes. Add additional water if needed for the enchilada sauce. Consistency should be like a gravy. Take off from heat and mix in sour cream.
Spread scant 1/4 cup sauce in bottom of 8" baking dish. Mix 1/4 cup sauce into vegetable mixture. Warm tortillas with a bit of oil to make pliable. Place generous 1/3 cup vegetable filling in center of 1 tortilla; top with 1 tablespoon grated cheese. Roll up to enclose filling. Place enchilada in baking dish. Repeat with remaining tortillas and filling. Cover enchiladas generously with sauce, then (optionally) sprinkle with remaining ~1/2 cup cheese.
Bake vegetable enchiladas until heated through, about 30 minutes.