Preheat the oven to 375 degrees F. Slice 6 of the tortillas into 1-inch strips, and then arrange them on a baking sheet in a single layer and spray with cooking spray. Bake until deeply golden, season with salt and reserve.
Heat the corn oil in a soup pot or large Dutch oven over medium-high heat. Chop the remaining 6 tortillas into 1 to 1 1/2-inch squares, and then stir into the pot 1 minute. Add the chili powder, cumin, garlic, bay leaves, onions and cilantro, sprinkle with some salt and pepper and stir to toast the spices. Cook until the onions are translucent, 12 to 15 minutes.
Pour in the stock with the tomatoes and chiles and bring to a boil. Add the chicken, reduce the heat a bit and simmer to cook through, 15 minutes. Remove the chicken, slice or chop the breasts, and then return to the soup. Remove the bay leaves before serving.
To serve, dice the avocados and cut the limes into wedges. Place some avocado at the bottom of each soup bowl and douse with a healthy squeeze of lime juice before filling the bowls with soup. At the table, top the soup with the baked tortilla strips, jalapenos, red onions, queso fresco and sour cream.
Cook's Note: Cool and store for a make-ahead meal. Bring to room temperature before reheating over medium heat.