Preheat oven to 400 degrees. Grease a small 1-1/2 or 2 quart baking dish.
Melt butter in a medium saucepan over medium heat. Add the onion and cook until softened, stirring occasionally, about 5-6 minutes. Add garlic and stir, cooking until fragrant, about 1 minute.
Add 1 1/2 C of the heavy cream, nutmeg and cayenne and bring to a simmer. Simmer, stirring occasionally, until slightly thickened, about 4-5 minutes.
Mix in the spinach, 1/2 tsp salt and 1/4 cup of the parmesan. Stir gently until blended.
Add the cream cheese and stir until melted. Season with more salt and pepper to taste. (Be sure to taste it here because spinach can be quite bland). If the mixture seems overly thick, you can add more cream at this point and cook to thicken.
Transfer the mixture into the baking dish, smooth the top, and sprinkle with the panko crumbs and the rest of the parmesan cheese.
Bake until bubbling and light golden on top, about 20 minutes.
Enjoy!
