Preheat oven to 350 degrees. In small saucepan over medium-high heat, melt butter until foaming. Add flour and cook 1 minute, stirring constantly.
Stir in enchilada sauce, chicken broth, and salt and pepper to taste (I like 1/4 teaspoon salt and 1/4 teaspoon pepper). Bring to boil; reduce heat and simmer uncovered until thickened, 10 to 15 minutes. Keep warm over low heat.
In medium saucepan or skillet, heat oil until shimmering. Add onions and green chiles and cook until softened, about 5 to 7 minutes. Stir in chicken and sour cream until heated through and creamy, about 3 minutes.
Meanwhile, on a microwave safe plate, wrap corn tortillas in damp paper towels. Microwave 30 to 40 seconds.
Spread 1/2 cup enchilada sauce in bottom of 8 1/2-inch by 11-inch baking dish. Working with 1 tortilla at a time, add 1/4 cup of creamy chicken filling to a warmed tortilla. Roll tortilla around filling and place seam-side down in baking dish. Repeat with remaining tortillas.
Pour remaining enchilada sauce over rolled enchiladas and sprinkle with Monterey jack. Bake until cheese is hot and bubbly, about 25 to 30 minutes. Remove from oven and let stand 5 minutes. Garnish with scallions and serve.
Red enchilada sauce: A 10-ounce can of any purchased brand, such as Siete, Old El Paso, Las Palmas, or Frontera, acts as the base of the vibrant enchilada sauce.
Chicken broth: Feel free to use Homemade Chicken Broth or your favorite store-bought brand. You'll just need 1 cup for this recipe to layer extra flavor into the enchilada sauce, so no one will likely notice the difference.
Chicken: 1 1/2 cups of cooked chicken is all you'll need for this enchilada recipe. Leftover chicken from earlier in the week or store-bought or Homemade Rotisserie Chicken. If you have time, whip up a batch Blackened Chicken to crank up the spice content.
Sour cream: Or plain full-fat Greek yogurt.
Corn tortillas: You'll wrap a stack of 6-inch corn tortillas in a damp paper towel (see Step 4) so they're pliable and easy to work with. You can also use flour tortillas if you prefer.
Yield: This Sour Cream Chicken Enchiladas recipe makes four two-enchilada servings, which makes for a satisfying main dish.
Storage: Store leftovers covered in the refrigerator for up to 4 days.
Make ahead: The enchilada sauce can be made 3 days in advance and the chicken filling can be made 1 day in advance.