In a large skillet over medium heat, melt 1 cup of butter. Add the chopped shallot and sliced mushrooms, sautéing until softened, about 5 minutes. Stir in the thawed and drained spinach, mixing well. Finally, add the cooked, shredded chicken and stir until combined. Remove from heat and set aside.
In the same skillet, melt another tablespoon of butter, then whisk in the flour, cooking for about 1 minute. Gradually whisk in the chicken broth, ensuring there are no lumps. Bring to a simmer and cook until the mixture thickens, about 5 minutes. Next, reduce the heat to low and stir in the sour cream, cream cheese, and green taco sauce until smooth and creamy.
Preheat your oven to 350°F (175°C). Spread a thin layer of the creamy sauce on the bottom of a 9x13 inch baking dish. Take a tortilla, scoop a generous amount of the chicken, spinach, and mushroom filling into the center, roll it up, and place it seam-side down in the baking dish. Repeat this process until all tortillas are filled and arranged in the dish.
Pour the remaining creamy sauce over the top of the assembled enchiladas, ensuring they are well-coated. Sprinkle the grated Monterey Jack cheese evenly over the top.
Cover the baking dish with aluminum foil and bake for 20 minutes. After that, remove the foil and bake for an additional 15 minutes or until the cheese is bubbly and golden.
