Using two forks, finely shred the chicken.
Oil a low-sided casserole dish and preheat oven to 350 F.
In the jar of a blender, combine the stock, chiles and adobo sauce, garlic, tomatoes, tomato paste, onion and 1 tsp of the salt and liquefy.
In a medium saucepan over medium-high heat, heat the oil until it's shimmering. Pour the contents of the blender over the hot oil and bring to a boil, then turn off the heat.
Ladle about 1/4 of the sauce over the bottom of the casserole dish. Add 4 tortillas to the bottom of the pan, arranging them in one layer. Distribute 1/4 of the chicken over the tortillas. Top with 1/4 of the cheese. Repeat to make two more layers of sauce, tortillas, chicken, and cheese, until you have used up all of the ingredients.
Top with nachos and finish with a sprinkle of mozzarella cheese.
Cover dish with foil and bake for 15 minutes. Then remove the foil and bake for another 5 minutes. As soon as it's bubbling on the sides and the cheese is golden brown, it's ready to eat!
Place a spoonful of sour cream and a scattering of cilantro on each portion. Serve hot and enjoy leftovers for up to a week.