Preheat the oven to 425°F (220°C).
Chop the spinach, dice the onion, finely chop garlic.
Heat the butter in a large frying pan over medium heat. Add the onion and cook for 4-5 minutes until soft and translucent. Add the garlic, cook for 30 seconds.
Add the spinach and cook until all the excess liquid is evaporated.
Transfer the spinach to a bowl, add the remaining ingredients except feta cheese (parsley, dill, nutmeg, lemon zest, cream cheese, parmesan). Stir everything together and season with salt and pepper to taste. Gently fold in crumbled feta cheese (you want it crumbled through the filling). Chill the filling until cold (10-15 minutes in the fridge should be enough).
Prepare the muffin pan: grease it generously with vegetable oil (do not use muffin liners).
Take the puff pastry out of the fridge, roll it out slightly into a rectangle (if it's already thin and in a shape of a rectangle you don't need to roll it out). Divide the rectangle into 12 squares (4 columns and 3 rows).
Place each pastry square in the center of each muffin cup.
Divide the cold filling between all the pastry cups.
Bring the four edges of each puff pastry square together towards the center and lightly pinch them together in the middle (no need to seal them completely).
Bake the pastries for 15 minutes then reduce the temperature to 320°F (160°C) and bake for 10-15 minutes or until deeply golden.
Take the muffin pan out of the oven, let cool slightly then run a knife or a small palette knife around each pastry cup to help release it from the pan.