Put chicken stock in a pot and bring to a slow boil. While the stock heats up, combine chopped onion, half of the cilantro leaves and chipotle in a food processor. It’s a good idea to start with one chili to test the heat — you can always add more later. Add a little stock and pulse until the mixture is a paste. Add the mixture to the pot of stock and bring to a boil. Remove from heat and add chicken.
Heat enough oil in a pan to cover a layer of tortilla strips. Cut corn tortillas into strips and add to heated oil a layer at a time. Do it in small batches or they’ll stick together and burn. When they are golden, remove from the oil and place on paper towels and salt while still hot. (If you have a fryer, use that instead.)
Assemble the soup one bowl at a time. Start with the avocado chunks, then sprinkle the shredded cheese over generously and add some chopped cilantro. Ladle the soup over the small stack and add the tortilla strips. The hot broth will melt the cheese into gooey goodness.