In a large sauté pan, melt the butter over medium heat. Add the garlic and cook for 30 seconds. Add the cream and bring to a boil. Lower the heat to medium and cook the cream at a slow boil, stirring occasionally until the cream has thickened and reduced by half, about 25 minutes. Season with kosher salt, black pepper and nutmeg.

Add half of the spinach to the cream sauce, tossing to coat as the spinach cooks down. Add the remaining spinach as the first batch is just beginning to wilt, then add the Parmesan cheese, toss with the spinach leaves, and cook until the last batch is barely done. The spinach will continue to cook once it's removed from the heat. Sprinkle with more Parmesan cheese as desired and serve warm.