Tear one corn tortilla into pieces and add to a blender with the crushed fire-roasted tomatoes, chicken broth, 2 chipotle peppers, 2 tablespoons chopped cilantro, 1/2 teaspoon cumin, 1/2 teaspoon salt, and 1/4 teaspoon black pepper; blend until very smooth.
Heat 1 tablespoon of the vegetable oil in a saucepan over medium heat, pour in the blended sauce, and simmer for about 8 minutes until slightly thickened; remove from heat and set aside.
Preheat the oven broiler. In a medium bowl, combine the shredded rotisserie chicken, 2/3 cup sour cream, 1/4 cup sliced green onion, and 1 cup of the shredded Mexican cheese; season with salt and pepper to taste.
Place the remaining 8 tortillas on a microwave-safe plate, cover with a damp paper towel, and microwave about 30 seconds to soften.
Grease a 9x13-inch baking dish. Spoon about 1/3 cup of the chicken mixture down the center of each softened tortilla and roll up; place seam-side down in the prepared dish.
Brush the tops of the rolled enchiladas with the remaining 1 tablespoon of vegetable oil and broil 3–5 minutes until the tortilla tops are lightly toasted.
Pour the warm chipotle-tomato sauce over the enchiladas and sprinkle the remaining 1 cup of shredded cheese on top.
Broil just until the cheese is completely melted and bubbly, then remove from the oven.
Garnish with chopped cilantro and additional sliced green onion, drizzle with sour cream, and serve with avocado slices.