Preheat the oven 350 degrees. Prepare two 9x13 casserole dishes with nonstick cooking spray.
In a large skillet or dutch oven over medium heat, add the olive oil and add the mushrooms and onions. 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, taco seasoning, and bell peppers. Cook for 2 to 3 minutes or until heated through.
Meanwhile, in a large bowl, stir together 2 cups of cheese, sour cream, heavy cream, and xanthan gum.
Add the contents of the skillet to the bowl and stir to combine.
Divide between the two casserole dishes and top each with the remaining cheese.
To bake, place the casseroles in the oven for 30 minutes.
Top with fresh cilantro and serve.