1To make the sauce, in a saucepan on medium-low, melt the butter then whisk in the flour until smooth; cook for 1 minute. Slowly add the chicken broth while whisking constantly; simmer until the sauce has thickened slightly, about 5 minutes. Put the sour cream, tomatillos, green chiles, cilantro, and cumin to a blender and puree until smooth. Add to the chicken broth mixture and blend to combine. Let simmer gently until warmed and thickened, about 5 minutes. Season to taste with salt and pepper.
2Preheat the oven to 375°F. Have ready a 9×13 dish. Spread 1 cup of the sauce into the bottom of the dish. Toss together the chicken, half the cheese, and 1/2 cup sauce in a bowl.
3In the cast iron pan, warm the canola oil over medium-high heat. Fry the tortillas quickly and set aside to drain on paper towels. Blot dry.
4For each enchilada, add some of the chicken mixture and roll up tightly. Place in the baking dish and repeat. (These can be prepped up to 1–2 hours ahead.) You should easily fit 12.
5Warm the remaining sauce. Pour the sauce over the enchiladas to generously coat them, then top with the remaining cheese. Bake, uncovered, until bubbly and heated through and the cheese is golden brown, 20 to 30 minutes. Let sit for 5 minutes before serving.