Step 1: Thaw the frozen spinach completely and squeeze out as much water as possible. This step is crucial for a creamy, not watery, final product. You can use a clean kitchen towel or cheesecloth to wring it out.
Step 2: In a large saucepan or Dutch oven, melt the butter over medium heat. Add the minced garlic and sauté for about 1 minute until fragrant, being careful not to burn it.
Step 3: Sprinkle the flour over the butter and garlic, whisking constantly for 1-2 minutes to create a roux. Cook until the raw flour smell disappears.
Step 4: Gradually whisk in the whole milk and heavy cream, a little at a time, until the sauce is smooth and lump-free. Continue to whisk as the sauce thickens, about 3-5 minutes. Bring to a gentle simmer.
Step 5: Reduce the heat to low. Add the softened cream cheese, 1/2 cup Parmesan cheese, nutmeg, white pepper, hot sauce, and salt. Stir continuously until the cream cheese is fully melted and incorporated, and the sauce is smooth and creamy.
Step 6: Stir in the well-drained spinach, ensuring it's evenly coated with the creamy sauce. Taste and adjust seasoning as needed.
Step 7: Transfer the spinach mixture to an oven-safe serving dish. If desired, sprinkle with additional Parmesan cheese. Bake in a preheated oven at 350°F (175°C) for 10-15 minutes, or until bubbling hot and lightly golden on top. Serve immediately.