Spinach Puffs

In a medium skillet, heat oil over medium heat. Add onions and season with salt. Cook, stirring occasionally, until softened, 4 to 5 minutes. Add garlic and cook until fragrant, about 30 seconds more. Remove from heat. In a large bowl, stir together cream cheese and 1 egg. Stir in garlic-onion mixture, spinach, feta, dill, and lemon zest and season with salt and pepper. Grease 9 cups of a muffin tin with cooking spray. On a lightly floured surface, roll out puff pastry sheet to a 12” square. Cut into 9 smaller squares, each about 4” wide. Ease each puff pastry square into a greased muffin cup, letting the corners hang over the sides. Add about 2½ tablespoons of filling to the center of each. Fold in pastry corners to meet in the center over the filling and pinch together. Refrigerate for 30 minutes (or freeze for 15 minutes). Meanwhile, adjust an oven rack to center position and preheat the oven to 400°. Beat remaining egg in a small bowl with 1 teaspoon water and brush on top of the chilled pastry.  Bake until puffed and golden brown on the bottom, about 25 minutes. Let cool slightly before lifting out of the muffin tin. Serve warm or at room temperature.