Add the spinach to a sieve set over a bowl and press out the juices with the back of a spoon. In another bowl, mix the cream cheese, garlic and egg together. Stir in the spinach and season with nutmeg, salt and pepper.
Unfold the pastry sheets and put one sheet on the work surface with the long edge facing you. Cover the remaining sheets with clingfilm so that they don't dry out. Brush the pastry in front of you with a little melted butter, then cut into 6 strips, 7.5 × 23 cm (3 × 9 inches). Put a teaspoon of the spinach mixture near the bottom right corner of each strip.
Holding the bottom right corner of one of the strips, lift and fold up and over diagonally to make a triangular shape to enclose the filling. Fold the bottom left corner straight upwards to make a thicker triangle, then continue folding until you reach the top of the pastry.
Make 5 more boureks, then take a second sheet of pastry and repeat. Continue until all the pastry and filling has been used up. Transfer to a baking sheet and brush with the remaining butter. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 15 minutes until golden. Leave to cool slightly, then serve warm or cold.