For the caramelized onions, melt the butter and sauté on low heat all the ingredients until tender, than blend all together. Next, cut the beef in 2 portions and seal on a well preheated saute pan, remove the beef and add the shredded onion, saute in butter until browned than add the sugar and caramelize. Now pour in the balsamic and wine, simmer until thickened. Peel and dice the potato, boil until softened than mix in the thyme and a knob of butter. Once fillet is almost cooked cover with the brie cheese and insert for a few minutes under the grill, serve with the liver pate on top.