1/2 cup butter
6 ounces cremini mushrooms, sliced
3/4 cup frozen chopped spinach, thawed and squeezed dry
1-2/3 cups heavy whipping cream
3 tablespoons cream cheese
3/4 cup grated Parmesan cheese
1 teaspoon garlic powder
1/4 teaspoon freshly grated nutmeg
Salt and freshly ground black pepper, to taste
1/8 - 1/4 cup grated Parmesan cheese, for garnish
Melt butter in a 10-inch skillet over medium heat. Stir in mushrooms and cook for 2 to 3 minutes. Add spinach and cook until warmed, about 1 minute. Add cream and cream cheese; cook and stir until cream cheese is melted, about 5 minutes.
Stir Parmesan cheese and garlic powder into spinach mixture; season with nutmeg, and salt and pepper. Simmer until sauce is thickened and smooth, about 10 more minutes. Serve immediately, topped with additional cheese, if desired.