Before you begin, preheat your oven to 350 and spray down a casserole dish with cooking spray. I use a 9 x 13 dish.
Chop or shred cooked chicken and divide between tortillas. Top chicken with cheese and roll up tortilla.
Place filled tortillas into pan, seam side down.
Melt butter in a pan on the stove.
Once butter is melted, add in 3 tablespoons of flour to make a roux.
Add in chicken broth and stir to combine.
Leave sauce on medium-high heat until sauce begins to thicken, which takes just a few minutes. Be careful the pan isn't too high on heat so the bottom doesn't burn.
Once white sauce is thickened, pull off heat and add 1 cup of sour cream. Whisk together.
Immediately pour sour cream cheese sauce over prepped tortillas, taking care to fully cover the tortillas. Its fine to use a spatula to spread sauce so it gets on the corners of the tortilla. Any exposed tortilla without sauce or cheese can harden in the oven.
Once tortillas are fully covered with white sauce, add rest of shredded cheese.
Place in oven for 20 minutes until heated, bubbly and golden brown.
Typically 2 chicken breasts, shredded or chopped, can easily be divided into the tortillas.