Melt butter in a medium saucepan over medium heat. Stir in the flour and cook 1 min
Slowly whisk in the heavy cream. Cook, whisking, until mixture thickens enough to coat a cooking spoon. Stir in the Greek yogurt, garlic powder and pepper. Cook 1 min., stirring constantly
Stir in the Parmesan and continue cooking, stirring constantly, until the cheese melts, about 30 seconds. Set aside
Thickly slice the cooked stuffed mushrooms, cutting through the filling. Transfer to a mixing bowl with the chopped spinach. Gently toss to combine. (It’s okay if the filling separates from the mushroom.)
Spoon the mixture into a buttered, 1-1/2-qt. baking dish. Pour sauce over the mushroom mixture and gently toss to coat. Sprinkle the top of the casserole with the panko crumbs.
Bake until the center of the casserole is hot and bubbly and the panko crumbs are golden brown, about 15 to 20 min
Remove from oven and let rest 5 min. before serving