Tie the pork fillet with kitchen string and cut it into 2,5 cm rounds; season with salt and pepper. Heat olive oil in a medium sauté pan, and sear with rosemary, until golden. Remove meat from pan and set aside; pour the wine in and let alcohol evaporate and bubble away. 

Peel and slice onions as thinly as possible.  In another sauté pan, heat additional olive oil.  When heated, add  onions with a pinch of salt.  Cook until onions are softened and translucent - depending on the variety used, the cooking time can take anywhere from 15-20 minutes or longer).  When onions begin to caramelize and become very soft, add sugar, balsamic vinegar and thyme. Continue caramelizing for several minutes.  

Prior to serving, finish cooking the meat by adding the sliced meat back into the sauté pan with the condensed wine sauce.  Allow to cook for 10-15 minutes.  The wine sauce can be thickened by adding a little butter and flour (cut a small piece of butter, roll it in the flour, and add to the wine sauce).  Keep stirring until sauce has thickened.    Cut the string off, serve meat rounds topped with the onions and a drizzle of the sauce on top.