Additional sour cream and cilantro, for serving (optional)
Preheat oven to 375°F. Grease a 9x13-inch casserole dish with butter.
Melt 2 tablespoons butter in a large skillet over medium high heat. Add chicken and season with salt and pepper. Brown chicken until cooked through, about 5 minutes. Remove chicken from skillet and set aside.
Melt remaining butter in the skillet and add onions and green chiles. Cook until onions have softened, about 4-5 minutes. Remove skillet from the heat and add chicken and avocado; set aside.
Whisk together enchilada sauce and sour cream in a medium bowl. Spread a small amount on the bottom of the prepared casserole dish.
Divide chicken mixture evenly among tortillas; roll and place seam-side down in casserole dish. Pour remaining sauce over the top of the tortillas and sprinkle with cheese.
Cover with foil and bake 20 minutes. Remove foil and bake an additional 10 minutes until lightly browned.