Peel and chop the onion and fry it in a pan with oil for ten minutes. Add the thyme and rosemary and cook for one minute. Add the broth and bring to a boil. Cook over low heat until have reduced completely, about 15 minutes.
Line the base of a 20 cm mold with parchment paper. Cut the potatoes, peeled, with the help of a mandolin, and we cover the base with a layer of them. Season to taste and cover with a thin layer of poached onion.
We repeat the operation finishing with a layer of potato. Cover with parchment paper, pressing firmly. We cover the mold with aluminum foil and take it to the oven, preheated to 200ÂșC with heat above and below, for -approximately- an hour and a quarter or until the potatoes are tender.
We then remove the mold from the oven, remove the aluminum foil and the vegetable and leave rest the millefeuille five minutes before unmolding. Turn it over onto a plate suitable for the oven and remove the parchment paper.
Put the millefeuille back into the oven, this time with the grill on, and place it on the upper part. Gratinate for a couple of minutes or until the surface looks golden. We serve immediately.