In a medium large saucepan heat the oil or lard over medium. Add the garlic and onion and cook, stirring regularly until golden. About 7 minutes.
Use a slotted spoon to scoop up the garlic and onion, pressing them against the side of the pan to leave behind as much oil as possible and transfer to a food processor or blender. Set the pan aside. Add the tomatoes to the garlic and onion and process to a smooth puree.
Set the saucepan over medium-high heat. Wen hot, add the puree and stir nearly constantly until it has thickened to the consistency of tomato paste, about 10 minutes. Add the broth and mushrooms and bring to a boil, them partially cover and gently simmer over medium to medium low heat for 30 minutes. Taste and season with salt, usually 1/2 teaspoon depending on the saltiness of your broth.
Heat oven to 375 degrees. Cut the tortillas in half, then cut crosswise into 1/4 inch strips. Spread the tortilla strips in a single layer on a baking sheet and spray or lightly brush with oil and toss to coat. Set in the oven and bake, stirring around every couple of minutes or so, until lightly browned and crispy. About 8 minutes.
Serving the soup. Cut open the chipotle chiles and scrape out the seeds. Cut the chiles into thin strips. In each soup bowl place a portion of the cheese and cubed avocado, a generous sprinkling of the watercress leaves and a few strips of the chipotle. Ladle the broth into the bowls and top each with a handful of crispy tortilla strips.