Melt the butter in a medium saucepan over medium heat. Add the garlic and onion and cook, stirring occasionally, until softened, about 6 minutes. Add the heavy cream, nutmeg and cayenne and bring to a simmer. Simmer, stirring occasionally, until slightly thickened, about 5 minutes. Mix in the spinach, 1/2 teaspoon salt and 1/4 cup of the Parmesan. Add the cream cheese and stir until melted. Season with salt and pepper to taste.
Transfer the mixture to a 1 1/2- to 2-quart baking dish and sprinkle with the panko and remaining 1/4 cup Parmesan. Bake until bubbling and light golden on top, about 15 minutes.