Cut the leek lengthwise, wash it thoroughly from the dirt and chop it into half-moon slices. Mince the garlic.
Wash the Swiss chard and drain it from the water. Remove the harder white parts of the stem (if they are very hard and sturdy) and cut the green part into 1-2 cm pieces.
In a pan, heat the olive oil, add minced garlic, and leeks and cook for a few minutes until everything softens. Add Swiss chard and cook for a few more minutes. Season with salt, freshly ground black pepper and freshly grated nutmeg. When the vegetables have softened, remove the pan from the heat, let everything cool down a bit, and stir in the ricotta cheese.
Preheat your oven to 400°F (200°C) and line two baking sheets with parchment paper. Lightly beat an egg for the egg wash. Also, prepare a small bowl with some water and a kitchen brush. You'll need that for sealing the turnovers.
Roll out the puff pastry (which has been previously thawed) and cut it into 12 squares (1 square = 9 cm/3,5 in). Take a piece of stuffing with a teaspoon, place it in the middle of the square, brush the two edges with some water and fold the pastry so it forms a triangle. Crimp the edges with the fork.
Transfer the turnovers to a baking tray, brush with egg wash, and sprinkle with some poppy or sesame seeds. Bake in preheated oven until golden brown, 20-25 min.
Allow to cool slightly and serve warm.