Rip one tortilla into pieces and add to a blender with the crushed tomatoes, chicken broth, chipotle peppers, chopped cilantro, cumin, 1/2 teaspoon salt, and 1/4 teaspoon black pepper; blend until smooth.
Heat 1 tablespoon vegetable oil in a saucepan over medium heat, add the blended sauce, and simmer 8 minutes until slightly thickened; remove from heat and set aside.
Preheat the broiler. In a medium mixing bowl, combine the shredded chicken, 2/3 cup sour cream, sliced green onions, and 1 cup of the shredded cheese; taste and adjust seasoning with salt and pepper if needed.
Place the remaining 8 tortillas on a microwave-safe plate, cover with a damp paper towel, and microwave about 30 seconds to soften them.
Grease a 9×13-inch baking dish. Spoon about 1/3 cup of the chicken mixture down the center of each tortilla and roll up; place seam-side down in the prepared dish.
Brush the tops of the rolled enchiladas with the remaining 1 tablespoon vegetable oil and broil 3–5 minutes until the tortillas begin to toast.
Pour the warmed chipotle-tomato sauce evenly over the enchiladas, sprinkle with the remaining 1 cup shredded cheese, and return to the broiler until the cheese is completely melted.
Sprinkle with chopped cilantro and additional sliced green onion, drizzle with extra sour cream if desired, and serve with avocado slices.
