In a small saucepan over medium heat add the olive oil. Sauté the onion for 3 minutes. Add garlic and sauté for one more minute. Reserve.
In a large bowl, combine the ricotta, mozzarella cheese, cream cheese, Parmesan cheese, Italian herbs, and pepper; mix well until smooth. Add onion mixture and spinach. NOTE: I did this step with the electric mixer, it's faster, less work, and the texture will be smoother.
Preheat oven to 350º F. Line a cookie sheet with parchment paper.
Roll out the puff pastry carefully. Using a cookie cutter (or a cup) cut 3.5-inch circles from the puff pastry. I got around 10 rounds out of each sheet.
Brush lightly each pastry round with milk. Top each round with a spoonful of the spinach mixture. Cover it with another round of pastry then crimp the sides with a fork.
Place the puffs on the baking sheet and brush with milk.
Bake for 15 - 20 minutes until golden brown and puffed.
Remove from the oven and allow cooling slightly. Serve.