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1
Preheat oven to 425 F. Spread the tortilla strips in a single layer on a rimmed baking sheet. Spray the strips with cooking spray, flipping to coat both sides, and sprinkle with salt. Bake for about 15 minutes, or until crisp, stirring the strips halfway through baking.
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2
Add the chicken broth, chicken breasts, 2 of the onion quarters, 2 garlic cloves, cilantro and oregano to a large Dutch oven. Bring to a simmer over high heat then cover, reduce heat to medium-low and cook until the chicken is cooked through, about 20 minutes. You can verify the chicken is cooked with an instant read thermometer - it should be at least 160 F.
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3
Transfer the chicken to a cutting board, and when it is cool enough, remove the meat from the bones and shred with two forks. Strain the liquid in the pot into a clean bowl - discard the solids.
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4
In the bowl of your food processor, process the last 2 onion quarters, 2 garlic cloves, tomatoes and chipotle chiles until smooth. Set your Dutch oven over high heat and add the oil. When the oil is very hot, add the pureed mixture and cook, stirring often, for about 10 minutes, or until the mixture darkens in color.
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5
Add the strained broth back into the Dutch oven with the pureed mixture and stir to combine. Cover the pot and simmer for 15 minutes. Add the shredded chicken to the pot and continue to cook until the chicken is heated through, about 5 minutes. Season to taste with salt and pepper.
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6
Place tortilla strips in the bottom of the bowls, then ladle soup over the top. Garnish and serve.
