In the bowl of your crockpot, combine the chicken, onion, enchilada sauce, chipotle, oregano, cumin, and salt. Cover and cook on LOW for 4-5 hours or on HIGH for 2-3 hours.
When finished cooking, shred the meat and toss with the sauce. Preheat the oven to 425° F.
Warm the tortillas for 30 seconds to 1 minute in the microwave, until pliable. On a baking sheet, rub the tortillas with olive oil. Lay each tortilla flat and then layer evenly with cheese and chicken. Fold the other half of the tortilla over the filling, gently pushing to adhere. Transfer to the oven and bake for 5-8 minutes, then flip and cook another 5 minutes, or until the cheese has melted and the tortillas are crisp.
Meanwhile, make the Avocado Crema. Combine all ingredients in a blender and blend until smooth and creamy. Season with salt.
Serve the tacos topped with avocado, lettuce, limes, and any other desired toppings.
