In a medium saucepan, melt the butter over medium heat. Whisk in the all-purpose flour and cook, stirring for 1 minute. Gradually add the chicken broth and cook, stirring until thick, which will take about 5 minutes.
Whisk in the cream cheese until melted. Stir in the sour cream and green taco sauce. Let cool.
Preheat the oven to 425℉. Spray a 13×9 inch pyrex pan with nonstick spray.
Lay out a tortilla on your work surface. Spread 2 tablespoons of Monterey Jack cheese down the middle, then add ¼ cup of chicken, a sprinkle of chopped shallots, a tablespoon of spinach, and a few slices of mushrooms. Spoon 2 tablespoons of the sauce on top of the mixture.
Roll the tortilla tightly and place it seam-down in the prepared dish. Repeat with the remaining tortillas.
Spoon the remaining sauce over the rolled enchiladas, then sprinkle with the remaining Monterey Jack cheese.
Bake for 25 to 30 minutes, or until lightly browned and bubbly.
