Preheat oven to 350°F.
Place oil into a large skillet over medium heat. Saute onion and jalapeno until softened, about 5 minutes. Stir in garlic and cook for 1 minute.
Add chopped spinach and cook until wilted down. Stir in chicken, salt, pepper, garlic salt, cumin, and diced tomatoes. Reduce heat to low and simmer.
To make the sauce, place soup, enchilada sauce, sour cream, milk, and salt into a large saucepan over medium heat. Stir until warmed through.
Ladle a small amount of sauce into the bottom of a large 9x13-inch baking dish just enough to cover the bottom. Layer the tortillas and then spread some of the chicken mixture on top, spreading evenly. Repeat until all of the chicken mixture is used, ending with a layer of tortillas.
Top evenly with the sauce and shredded cheese. Bake for 25-30 minutes until hot and bubbly.