Prepare the filling: Heat olive oil in a skillet over medium heat. Add the chopped onion and garlic and cook for 2-3 minutes until softened and fragrant. Add the thawed spinach, breaking it apart with a spoon, and cook for an additional 2-3 minutes to heat through. Remove from heat and let it cool slightly.

Make the dip mixture: In a large mixing bowl, combine the cream cheese, sour cream, mayonnaise, mozzarella cheese, parmesan cheese, and the spinach mixture. Stir until smooth and well-combined. Season with salt and pepper to taste.

Prepare the pastry shells: If using phyllo dough, brush a sheet of phyllo with melted butter, then layer with 2 more sheets, brushing each layer with butter. Cut the layered phyllo into 3-inch squares. If using puff pastry, roll out the dough and cut it into 3-inch squares or circles.

Assemble the bites: Place a spoonful of the spinach dip mixture in the center of each pastry square. Gently fold the corners up to form a little pouch or cup. Press the edges to seal the filling inside. If using phyllo dough, you can also fold the edges in to create a little triangle.

Bake: Preheat your oven to 375°F (190°C). Place the spinach dip bites on a baking sheet lined with parchment paper. Brush the tops with a little more melted butter. Bake for 12-15 minutes, or until the pastry is golden and crispy.

Serve: Once baked, remove from the oven and allow the bites to cool for a few minutes before serving. Enjoy these delicious spinach dip bites warm, with a side of ranch dressing or marinara sauce if desired.