Step 1: Preheat and Prepare
Preheat your oven to 375°F (190°C). Lightly grease an 8×8-inch or 9×9-inch baking dish with butter or non-stick cooking spray. Set aside.

Step 2: Prepare the Spinach
This is the most important step! Place the thawed frozen spinach in a fine-mesh strainer or clean kitchen towel. Squeeze firmly to remove as much liquid as possible. You should be able to extract at least ½ cup of liquid. Really wring it out – excess moisture will make your dip watery and ruin the texture. Set the squeezed-dry spinach aside.

Step 3: Mix the Creamy Base
In a large mixing bowl, combine the softened cream cheese, sour cream, and mayonnaise. Using an electric hand mixer or a sturdy spoon, beat together until completely smooth and well combined with no lumps of cream cheese remaining. This creates the creamy foundation of your dip.

Step 4: Add Cheese and Seasonings
Add 1 cup of the shredded mozzarella cheese (reserving ½ cup for topping), all of the Parmesan cheese, minced garlic, diced onion, salt, black pepper, red pepper flakes (if using), and Italian seasoning (if using). Mix everything together until evenly distributed throughout the creamy base.

Step 5: Fold in Spinach
Add the squeezed-dry spinach to the bowl. Using a rubber spatula, fold the spinach into the cream cheese mixture, making sure it’s evenly distributed throughout. The spinach should be well incorporated with no large clumps.

Step 6: Transfer to Baking Dish
Spread the spinach dip mixture evenly into your prepared baking dish, smoothing the top with a spatula. Sprinkle the reserved ½ cup of mozzarella cheese evenly over the top. This creates a beautiful golden, bubbly cheese layer when baked.

Step 7: Bake Until Bubbly
Place the baking dish in the preheated oven and bake for 20-25 minutes, or until the dip is hot throughout and the cheese on top is melted, bubbly, and lightly golden around the edges. You should see the edges bubbling up.

Step 8: Optional Broiling
For extra golden, slightly crispy cheese on top, turn on your broiler and broil for 1-2 minutes, watching very carefully to prevent burning. The top should turn a deeper golden brown with some crispy spots.

Step 9: Cool and Serve
Remove from the oven and let the dip cool for 5 minutes before serving – it will be extremely hot right out of the oven. Serve warm with tortilla chips, crackers, bread cubes, pita chips, or fresh vegetables.