Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper.
On medium heat, sauté your chopped onion in a bit of olive oil spray until slightly browned. Turn off the heat and let it cool while you mix together the rest of the filling.
In a mixing bowl, combine chopped spinach, lemon zest, a squeeze of lemon juice, cottage cheese, mozzarella cheese, Parmesan cheese, minced garlic, cooled sautéed onion, salt, and pepper. Mix well until all ingredients are evenly incorporated.
Roll out the puff pastry sheet on a lightly floured surface and cut it into 6 equal squares.
Spoon the spinach and cheese mixture onto one half of each pastry square, leaving a small border around the edges.
Fold the pastry over the filling to create a small square shape and crimp the edges with a fork firmly to seal.
Place the hand pies onto the prepared baking sheet and bake in the preheated oven for 20-25 minutes, or until the pastry is golden brown and crispy.
Remove from the oven and let them cool slightly before serving.