Preheat the oven to 400º F. Grease 2 regular size muffin tins.

In a large sauté pan over medium heat render the bacon until completely crisp. Transfer the bacon to a paper towel lined plate and add the onion and garlic to the pan. Cook the onion until tender, about 3 minutes. Add the spinach to the pan and continue to cook for 2 more minutes. Remove from the heat and season with salt and pepper. Add the herbs, beaten egg, olive oil, lemon juice and crumbled feta and mix well.

Allow the puff pastry to defrost. On a lightly floured surface, gently roll out 1 sheet of the puff pasty. Using a pairing knife cut the pastry into 9 squares. Brush the pastry with the melted butter and sprinkle with parmesan cheese. Line one muffin tin with the pastry squares and spoon 2 tablespoons of the spinach mixture into each cup. Bring the 4 points of the pastry together over the filling and gently press together. Repeat with the remaining puff pastry and fillling.

Transfer to the oven and bake for 15 to 18 minutes or until golden brown. Let the tarts cool in the pan on a wire rack for 10 minutes. Run a knife around the edges to loosen them and gently pop them out of the pan.

Serve immediately.