Preheat the oven to 180°C. Finely chop the onions, slice the leeks in half lengthwise then cut into thin strips. Roughly chop the chestnuts and the thyme finely. Sauté the onions in a little butter. Add the leeks and chestnuts and cook for 3 more minutes over a low heat.
Mix the goat’s cheese with 2 eggs, the honey and thyme, then add the braised vegetables and season with salt and pepper. Distribute the mixture on the puff pastry leaving a border of approx. 4 cm. on one long side and both short sides.
Separate the yolk of the remaining egg from the white, beat the yolk with a whisk and brush the edges of the pastry. Roll the strudel and brush it with the remaining yolk, then bake in the oven for about 25 minutes until the strudel is golden brown.