Leek and cheese tart with mastiha flavor

Preheat the oven to 190°C. Place the sheet of fresh puff pastry into the tart pan (a standard size pan, about 25 cm in diameter). Gently press the pastry onto the pan. Use a sharp knife to trim any excess edges. Prick the bottom of the pastry with a fork and place the prepared pan in the refrigerator for 20 minutes.
Meanwhile, prepare the filling: Heat the butter in a pan and lightly sauté the leek until it softens slightly, brightens in color, and becomes a little glossy. Season with salt and pepper lightly. It should not become too soft.
Blend the three cheeses in a blender along with the egg and cream. Spread the mixture over the tart and add the sautéed leek rings. Sprinkle with nutmeg and the remaining grated feta. Bake the tart for about 25-30 minutes, or until the puff pastry rises and turns golden, and the cheese mixture sets. Let it cool and serve.