Wash the meat and cook in boiling broth for about 20 minutes. In the meantime, place 12 puff pastry slices next to each other and defrost for about 20 minutes. Refreeze the rest of the puff pastry. Clean, wash and cut the leek into rings. Melt the fat in a pot.
Steam the leek for 4-5 minutes. Let it cool down on a flat plate. Drain the chicken filet, let it cool down briefly and pluck into pieces. Mix mustard, leek, meat and crème fraîche. Season with salt and pepper
Cut out 12 circles from the puff pastry with a cutter (13 cm Ø). Place 6 circles on a baking tray lined with baking paper. Whisk the egg yolks and brush the circles with them. Place 3 tablespoons of the filling in the middle of each of the 6 pastry circles and cover with another pastry circle. Press the edges well with a fork.
Brush the puff pastry pockets with the remaining egg yolk. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 40 minutes. If necessary, cover with aluminium foil after approx. 30 minutes. Arrange puff pastry pockets on plates