Preheat the oven to 350°F.
Thaw the frozen spinach and squeeze out excess water.
In a large skillet, sauté the chopped onion and minced garlic over medium heat until softened.
In a medium saucepan, make a white sauce by melting the butter over medium-low heat, then whisking in 1 tablespoon of all-purpose flour to make a roux, cooking for 1 minute.
Gradually pour in the heavy cream, whisking continuously to avoid lumps, and bring the mixture to a simmer.
Reduce the heat to low and stir in the softened cream cheese until fully incorporated and smooth.
Add the grated Parmesan cheese, nutmeg, salt, and pepper to the sauce, stirring until the cheese is melted and the sauce is well combined.
Add the cooked spinach to the cheese sauce and stir until the spinach is fully coated.
Transfer the creamed spinach to a baking dish and bake in the preheated oven for 15-20 minutes, or until heated through and lightly golden on top.