In a large Dutch oven or heavy-bottomed saucepan, melt the butter over medium-high heat. Add the minced shallots and garlic, stirring frequently until softened and aromatic, roughly 2 minutes.

Add the spinach in batches, incorporating each addition fully before adding more. Use tongs or a rubber spatula to fold and wilt the spinach until it’s all incorporated.

Sprinkle the flour over the wilted spinach, stirring continuously until no dry flour remains visible. Slowly pour in the heavy cream and milk, stirring gently to combine. Bring the mixture to a simmer, then reduce the heat to the lowest setting. Continue cooking, stirring occasionally, until the spinach is completely tender and the sauce has thickened to a creamy consistency, about 1 1/2 hours.

Season the spinach with freshly grated nutmeg and stir in the crème fraîche. Adjust the taste with salt and black pepper as needed, then remove the pot from the heat.

If desired, preheat the broiler to high. Transfer the creamed spinach to a 1-quart baking dish and sprinkle the grated Parmigiano-Reggiano evenly over the top. Broil the dish until the cheese forms a bubbly, golden-brown crust, approximately 2 minutes. Serve warm and enjoy the rich, creamy flavors.