Step 1: Preheat Oven and Prepare Ingredients
Preheat oven to 375°F (190°C). Lightly grease a 9-inch pie dish, 8×8-inch baking dish, or oven-safe skillet with cooking spray or butter. Thaw frozen spinach completely, then place in a clean kitchen towel or cheesecloth and squeeze out ALL excess water (this is crucial – you should get at least ½ cup of water out). Set aside.

Step 2: Sauté Aromatics
In a small skillet over medium heat, add olive oil or butter. Add diced onion and sauté for 3-4 minutes until softened and translucent. Add minced garlic and cook for 1 minute more until fragrant. Remove from heat and let cool slightly.

Step 3: Mix Creamy Base
In a medium mixing bowl, combine softened cream cheese, sour cream, and mayonnaise. Using a wooden spoon or hand mixer, beat until smooth and creamy with no lumps. This creates the luscious base for your dip.

Step 4: Add Cheeses
To the cream cheese mixture, add 1 cup of shredded mozzarella cheese and ¼ cup of grated Parmesan cheese (reserve the rest for topping). Mix until well combined.

Step 5: Fold in Vegetables
Add the squeezed-dry spinach, chopped artichoke hearts, sautéed onion and garlic mixture, salt, black pepper, and red pepper flakes (if using). Fold everything together until thoroughly combined and vegetables are evenly distributed throughout the creamy cheese mixture.

Step 6: Transfer to Baking Dish
Spread the spinach artichoke mixture evenly into your prepared baking dish. Smooth the top with a spatula. Sprinkle the remaining ½ cup mozzarella cheese and ¼ cup Parmesan cheese evenly over the top.

Step 7: Bake
Bake in preheated oven for 20-25 minutes, until the dip is hot and bubbly throughout and the cheese on top is melted and golden brown. For extra golden top, broil for 1-2 minutes at the end (watch carefully to prevent burning).

Step 8: Rest and Serve
Remove from oven and let rest for 5 minutes before serving (this allows it to thicken slightly and prevents mouth-burning). Garnish with fresh herbs if desired. Serve hot with your choice of dippers – tortilla chips, pita chips, bread, crackers, or fresh vegetables.