Heat the oil in a 2-quart or larger pot over medium-high heat. Stir in the flour and cook for 1 minute. Stir in the chili powder, oregano, cumin, and salt, and cook for 30 seconds longer. Stir in the chicken stock and tomato sauce and bring to a boil. Reduce the heat to low and simmer for 15 minutes, until the flavors are blended. Set the enchilada sauce aside to cool. 2. In a medium bowl, combine the chicken with the cheddar, Monterey Jack, and chopped onion.

Add 1 cup of the enchilada sauce to a 6-quart slow cooker and top it with 4 tortillas (they will slightly overlap). Add 2 cups of the chicken-cheese filling, spreading it evenly, and top with 4 more tortillas. Continue to build two more layers of sauce/filling/tortillas. Top the final layer with 1 cup of the sauce. Cook on high for 2 hours. During the last 10 minutes of cooking, pour another ½ cup of sauce over the top. Shortly before serving, garnish the enchilada with the Cotija, green onions, and cilantro. Allow it to sit for 10 minutes. Then cut it into individual portions and serve with the remaining sauce and the sour cream alongside.