Line your two trays with baking paper and preheat your oven to 210 degrees.
Place the ricotta, feta, grated parmesan cheese, baby spinach leaves, 1 egg, nutmeg, onion flakes and salt and pepper into a large bowl and stir until all of the ingredients have combined.
Break the extra egg into a small bowl and lightly whisk.
Place the partially thawed pastry sheets onto your bench and cut in half diagonally and then again so that you have four triangles.
Brush each edge of the pastry with a little of the egg before adding approximately 2 tablespoons of the three cheese filling to the triangle. Try to arrange your three cheese filling so that when you fold the pastry over to cover it, the filling is spread out evenly.
Fold the pastry in half to enclose the filling and use the edge of a fork to push the open sides of the pastry together.
Brush with a little of the egg and sprinkle with poppy seeds if desired.
Repeat process until all the pastry and three cheese mixture has been used - you may find you have enough filling leftover for a fourth sheet of pastry.
Place the Spinach and Three Cheese Triangles into the oven to cook for 25 minutes or until the pastry is golden and crispy.