Thaw the frozen spinach completely. Place the thawed spinach in a colander, fine-mesh sieve, or cheesecloth and press firmly to squeeze out as much liquid as possible. You can also use your hands. This step is critical to prevent a watery final dish. Set the dry spinach aside.

In a large skillet or sauté pan, melt the butter over medium heat. Add the minced shallot and cook, stirring occasionally, until softened and translucent, about 3-4 minutes.

Stir in the minced garlic and cook for another minute until fragrant, being careful not to burn it.

Sprinkle the all-purpose flour over the cooked shallots and garlic. Whisk constantly for 1-2 minutes to cook out the raw flour taste. This forms a roux which will thicken the sauce.

Slowly pour in the heavy cream and whole milk while whisking continuously to prevent lumps. Continue to cook, stirring frequently, until the sauce comes to a gentle simmer and has thickened enough to coat the back of a spoon, about 4-5 minutes.

Reduce the heat to low. Stir in the grated Parmesan cheese, freshly grated nutmeg, kosher salt, black pepper, and optional cayenne pepper. Continue stirring until the cheese is completely melted and the sauce is smooth.

Add the squeezed-dry spinach to the cream sauce. Stir everything together until the spinach is well-coated and heated through, about 2-3 minutes.

Taste the creamed spinach and adjust seasoning with more salt and pepper if needed. Serve immediately while hot.