In a large pan over medium to medium-low heat, saute the mushrooms in olive oil until they soften and release their moisture.Transfer the cooked mushrooms and spinach in a 1.5 qt baking dish.Using the same pan, heat the heavy cream, greek yogurt, pepper, and garlic powder over medium to a gentle simmer.Add the Parmesan cheese and stir frequently to thicken. If using xanthan gum, sprinkle over the sauce and continue to stir. About 5 minutesPour sauce over the mushrooms and spinach and gently toss to coat.In a small bowl, combine the breadcrumb toppings and layer ¾ths over the casserole.Cover with foil and bake at 400 for 20 minutes.Remove foil, add the remaining breadcrumbs and bake until golden brown. *I broiled mine for 5 minutes for an extra crisp!