1teaspoonchopped dill (I omitted because I dislike dill)
Kosher salt and freshly ground black pepper
1sheet frozen puff pastry (from a 17.3 ounce package), thawed, rolled out into a 12" square, kept chilled
Preheat oven to 400°F. Spray a standard size 6-cup muffin tin with nonstick baking spray.
Squeeze out as much water from the thawed spinach as possible, too much water will make soggy puffs. Mix the spinach with the feta, onion, garlic, olive oil, and dill (if using). Season to taste with salt and pepper. In a small bowl beat the egg to blend before gently stirring into the spinach mixture.
Cut the puff pastry into 3 equal strips. Reserve 1 strip for some other use (such as Nutella Palmiers). Cut each remaining strip into 3 squares for a total of 6. Place a square in each muffin cup, pressing into the bottom and up the sides, leaving the corners pointing up. Divide the filling evenly among the pastry cups. Fold pastry over filling, pressing the corners together to meet in the center. The pastry cups can be covered and refrigerated up to 3 hours ahead of time.
Beat the remaining egg to blend in a small bowl. Brush the pastry with the egg wash. Bake until the pastry is golden brown and puffed, about 25 minutes. Transfer the muffin tin to a wire rack to let the puffs cool in the tin for 10 minutes. Run a sharp paring knife around the edges of the puffs to loosen before turning the puffs onto the rack to cool slightly before serving.