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 minutes
Pour 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!
Sauteing mushrooms over medium to medium-low heat until they fully release their liquid is the single step that dictates whether this dish succeeds or turns watery. Mushrooms are roughly ninety percent water by weight. If that moisture is not driven off entirely in the pan before the vegetables reach the baking dish, it will pool in the cream sauce during baking and break the emulsion. Cooking the sliced fungi in olive oil requires patience, as salt draws out the water and high heat can brown the exteriors before the interiors soften. Once the mushrooms yield their liquid and shrink visibly, transfer them alongside the rough-chopped spinach to a 1.5 quart baking dish. Building the sauce demands equal vigilance. Heavy cream and Greek yogurt must be maintained at a gentle simmer rather than a rolling boil; excessive heat causes dairy proteins to curdle rather than smooth out. Incorporating real Parmesan cheese requires constant stirring to ensure the solids melt evenly into the fat without clumping at the bottom of the pan. Adding xanthan gum acts as an insurance policy against separation, though dusting it slowly prevents gelatinous lumps from forming. When combining the cream mixture with the vegetables, toss gently to avoid bruising the spinach leaves. Covering the baking dish with foil for the first twenty minutes at 400 degrees Fahrenheit traps steam to cook the interior evenly, while removing the cover for the final five minutes under the broiler creates a contrasting crisp crust. Substituting finely shredded mozzarella for part of the Parmesan changes the melting dynamics, yielding longer cheese pulls but introducing more moisture if the ratio exceeds fifty percent.