Boil chicken with celery and bullion for 15 minutes, set aside to cool, then shred.
Bring enchilada sauce to a simmer in a sauce pan, cover, and reduce heat. Simmer the sauce, stirring occasionally while you boil and shred the chicken.
Heat olive oil in a frying pan or skillet, add shredded chicken and fry for about 5 minutes. Then add around 2 cups of enchilada sauce to the chicken, cook for an additional 5 minutes.
Remove chicken and enchilada sauce from heat and start to build the enchilada bake.
Add enchilada sauce to the bottom of the baking dish. Dip each corn tortilla in the enchilada sauce, then add them to the dish in an overlapping fashion.
Add a layer of shredded chicken, then a layer of shredded cheese, then add a few additional spoonfuls of sauce.
Build an additional layer following the previous steps.
Top the casserole with one last layer of tortillas, spoon over any additional enchilada sauce, then top with more shredded cheese. Top the baking dish with tin foil.
Preheat the oven to 425 degrees Fahrenheit. Bake the enchiladas covered for 25 minutes, remove foil, bake an additional 10 minutes.
Remove the dish from the oven and let cool a few minutes before serving.
Serve in individual squares topped with shredded lettuce and sour cream.