Heat the oven to 425°F. Put the tomatoes and onion on a baking sheet, drizzle with 1 tablespoon oil, and roast for 20 minutes, until the tomato edges brown. Add the garlic for the final 8 minutes so it softens without burning.

Toast the ancho chile in a dry skillet for about 20 seconds per side, just until fragrant. Cover with hot water for 10 minutes, then drain.

Blend the roasted vegetables, softened chile, torn tortillas, and 1 cup broth until smooth. Work carefully with hot ingredients and vent the blender lid.

Warm the remaining tablespoon of oil in a large pot over medium heat. Add the purée and cook, stirring, for 5 minutes. Add the remaining 5 cups broth, chicken thighs, salt, and oregano. Bring to a gentle simmer and cook 18 to 22 minutes, until the chicken reaches 165°F.

Remove the chicken, shred it, and return it to the pot. Simmer 5 minutes more and adjust the salt. If the broth is too thick, add a little water or broth.

Meanwhile, spread the tortilla strips on a lightly oiled baking sheet and bake at 425°F for 8 to 12 minutes, turning once, until crisp. Serve the soup with strips, avocado, cilantro, lime, and optional queso fresco.