Preheat the oven 350 degrees. Prepare a 9x13 casserole dish with nonstick cooking spray.
In a large skillet or dutch oven over medium heat, add the olive oil and add the mushrooms, onions, and bell pepper. Cook until the onions are softened, about 5 minutes.
Add the chicken stock and cream cheese, stirring until the cream cheese is completely melted.
To the skillet, add the tomatoes and taco seasoning. Cook for 2 to 3 minutes or until heated through.
Meanwhile, in a large bowl, stir together 1 cup of cheese, sour cream, heavy cream, and xanthan gum.
Add the contents of the skillet to the bowl and stir to combine.
Transfer to the casserole dish and top each with the remaining cheese.
To bake, place the casserole in the oven for 30 minutes.
Top with fresh cilantro and serve.