How to Make It
In a saucepan, cook the onions over low heat with the butter and sugar until caramelized, 30 minutes. Add 3/4 cup of the wine along with the raspberry vinegar and boil until the liquid is syrupy, 8 minutes. Season with salt.
Meanwhile, season the pork with salt and pepper; sprinkle with the rosemary. Top each medallion with a slice of pancetta and tie together with string. In a skillet, heat the oil and garlic. Add the pork, pancetta side down, and cook over moderately high heat until browned, 3 minutes. Reduce the heat to moderate, turn the medallions and cook until just cooked through, 3 minutes. Transfer to a platter and tent with foil. Discard the garlic.
Add the remaining 1/3 cup of wine and the balsamic vinegar to the skillet and boil, scraping up the browned bits, until reduced by half, 2 minutes. Add the stock and simmer until reduced to 2/3 cup, 5 minutes. Stir in any pork juices and season with salt and pepper. Remove the string and set the pork on plates. Serve with the onion marmalade and sauce.