Step 1: If using fresh spinach, wash thoroughly and pat dry. If using frozen spinach, ensure it's completely thawed and squeezed of all excess water. In a large, heavy-bottomed pot or Dutch oven, melt 2 tablespoons of unsalted butter over medium heat. Add the minced garlic and sauté for 1 minute until fragrant. Be careful not to burn.
Step 2: Add the fresh spinach (or thoroughly drained frozen spinach) to the pot in batches, if necessary. Cook, stirring frequently, until the spinach has completely wilted and released its moisture, about 5-7 minutes. Remove the spinach from the pot and place it in a colander. Press out as much excess liquid as possible using the back of a spoon or your hands. Set aside.
Step 3: Return the pot to medium heat. Add the remaining 2 tablespoons of unsalted butter. Once melted, sprinkle in the all-purpose flour. Whisk continuously for 1-2 minutes to create a roux. Gradually whisk in the heavy cream and whole milk, stirring constantly to prevent lumps. Bring the mixture to a gentle simmer, allowing it to thicken slightly, about 3-5 minutes.
Step 4: Reduce the heat to low. Add the softened cream cheese, grated Parmesan cheese, and grated Gruyère cheese (if using). Stir constantly until all the cheeses are completely melted and the sauce is smooth. Stir in the freshly grated nutmeg, salt, and black pepper to taste.
Step 5: Return the well-drained spinach to the pot with the creamy cheese sauce. Stir everything together until the spinach is thoroughly coated and heated through. Cook for another 2-3 minutes, allowing the flavors to meld. Taste and adjust seasonings if needed. Serve your creamed spinach recipe hot!