For hash in a jar, first preheat the oven to 180 °C top/bottom heat or 160 °C hot air. Trim the meat and cut into very small cubes. Finely chop the herbs. Whisk the egg yolk with a little water.
Roll out the puff pastry and cut out 6 circles larger than the jars you are using. Brush the puff pastry with the egg yolk.
In a pan, sauté the vegetables in the oil and then add the lid on top. Saute for about 5 minutes over low heat until soft. Season with herbs, salt and pepper.
Fill the jars with meat, vegetables and then meat and vegetables again, one after the other. Finally, put a slice of puff pastry on top.
Put the hash in the jar in the oven for about 10 minutes.