Melt the butter in a large skillet over medium heat. Add the diced onion and cook until softened, about 5-7 minutes. Sprinkle in the gluten-free flour and cook for 1 minute, stirring constantly.

Gradually whisk in the chicken stock and milk until smooth. Bring the mixture to a gentle simmer, stirring occasionally, until it thickens. Season with kosher salt and pepper.

Stir in the salsa verde and sour cream. Continue to simmer for 2-3 minutes, allowing the sauce to heat through and flavors to meld.

Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.

Place a small amount of the sauce in the bottom of the prepared baking dish. Fill each tortilla with shredded chicken and a spoonful of sauce, then roll tightly and place seam-side down in the baking dish.

Pour the remaining sauce over the rolled enchiladas, ensuring they are well coated. Bake for 20-25 minutes, or until bubbly and heated through. Serve immediately.