Preheat broiler to high of 450° F with oven rack in top position. Line a sheet pan with aluminum foil. Place onion slices on sheet pan, sprinkle chopped chayote on the onions, and place whole peppers over the top. Broil veggies 8 minutes, or until peppers are blackened, turn peppers over, and broil another 8 minutes, until blacked. Remove tray from oven, and tightly cover with aluminum foil. Let stand 15 minutes or until cool enough to handle. Peel, seed, devein, and slice chiles into strips. Place cooked veggies in a large bowl, and set aside.
Taco filling can be made ahead to this point.
Heat a large dry cast iron skillet over medium-high heat. Add corn to skillet, and cook until starting to brown, about 10 minutes. Toss in broiled veggies, garlic, and salt, stirring to combine. Heat through, about 5-10 minutes. Stir in crema or sour cream, and cook 1 minute until heated through. Serve hot.