Start by giving the tomatoes a good rinse. I usually de-stem them as well. Roast the tomatoes in the oven (400F) for 20 minutes or until you need them.
Roughly chop the onion and peel 4 garlic cloves. Cook the onion and garlic in a glug of oil over medium heat, stirring regularly. I usually leave the garlic cloves whole knowing that all of this will be blended eventually.
When the onion is starting to brown add the onion-garlic mixture to a blender along with the roasted tomatoes and a single chipotle in adobo. Combine well. Note: a single chipotle will give the soup a whisper of heat, but feel free to adjust to your palate. I usually scrape out the seeds of chipotles but this is optional.
Add the blender mixture back to the pan that you used to cook the onion (I used a 2 quart saucepan for this recipe). Simmer this mixture over medium heat for few minutes, stirring regularly. Add the remaining ingredients: 6 cups of stock, 1 teaspoon Mexican oregano, 1 teaspoon salt, and some freshly cracked black pepper. Combine well. Bring this mixture to a boil, then lower heat, cover, and let simmer for 10-15 minutes.
As the soup simmers you can make the crispy tortilla strips! Chop up 6-8 corn tortillas into slices. Add a few tablespoons of oil to a frying pan over medium-high heat. When the oil is up to temp (360F or when a single strip sizzles heavily) you can add the tortilla strips. Cook for 30-60 seconds or until they are starting to turn darker in color, stirring regularly. Add the cooked tortilla strips to a paper towel lined plate to let them cool. Tongs work well for this step.
Take a final taste of the soup for seasoning. I added another generous pinch of salt, but keep in mind that the exact salt level will depend on which stock you're using.
Add 1-2 cups of the Tortilla Soup to a bowl and load it up with the crispy tortilla strips! I also topped with thinly sliced cabbage (tossed in lime juice and salt), serrano slices, freshly chopped cilantro, and a final squeeze of lime. Yum :)
Store soup leftovers in the fridge where it will keep for a few days. And if you have extra tortilla strips store them separately in a baggie in the fridge.