*The jus is the natural juices given off by roasted meat. It can be substituted for reduced unsalted chicken or beef stock; bring to boil 1 qt/1L stock and reduce to 1 cup/250ml. Add a dash of Worcestershire sauce to boost the color. In a saute pan sweat the chopped onion along with the bay leaf and olive oil for 15 minutes on low heat without browning. Add a splash clarified butter if needed. Add garlic and deglaze with the wine and reduce to dry. Add the meat jus, or reduced stock and cook 10 minutes on low heat.
Line a 9x1.5 inch (23x4cm) greased round cake pan with round parchment paper. Peel potatoes and soak them in cold water until ready to use. With a sharp chef knife, or a mandoline slicer, cut potatoes into 2 millimeter slices. Cut some out in round shape using a 1.5''/4cm diameter cookie cutter. Overlap them in circle on the bottom and the sides of the pan and brush with butter. Add layers of potato slices and trimming, jus mixture, pepper, herb and grated parmesan. Repeat these steps until done. Seal the dish with a greased parchment and foil.
Preheat oven to 350ºF/180ºC. Bake potato gratin for 90 minutes on a baking tray. Remove foil and parchment and bake 20 to 30 minutes more or until a toothpick inserted in the center of the gratin will come out clean. Let cool completely and refrigerate overnight.
Flip the gratin on a plater. Heat up the bottom of the cake pan using a blow torch or on the stove; remove the pan carefully. For a glossy finish; brush the gratin with olive oil if desired. Serve hot or chilled – Enjoy!