Preheat oven to 350 and grease a 9x9 baking dish. Set aside
Combine shredded chicken and 1/2 cup of the cheese and fill the 3 tortillas evenly. Place them in the greased baking dish
In a medium saucepan over medium heat, melt the butter. Whisk in the flour and let cook for one minute, stirring with the whisk. Add the broth and continue stirring for another 5 minutes or so until the mixture is thickened and bubbling up
Remove the saucepan from the heat and stir in the salsa. Once the mixture has slightly cooled, stir in the sour cream. Don't do this until the mixture has cooled slightly so the sour cream does not curdle
Pour the sauce over the prepared tortillas and top with the remaining cheese
Bake for 25 minutes then broil for 2-3 minutes to brown the top