In a small bowl or cup, mix together the rosemary, basil, thyme, salt and pepper. Rub this mixture onto the lamb chops on both sides. Place them on a plate, cover and set aside for 15 minutes to absorb the flavors.
Heat olive oil in a large frypan over medium-high heat. Place lamb chops in the frypan, and cook for about 3 1/2 minutes per side for medium rare, or continue to cook to your liking. Remove from the pan, and keep warm on a plate.
Add onions to the pan, and cook for a few minutes, just until browned. Stir in vinegar, scraping any bits of lamb from the bottom of the frypan, then stir in the chicken stock. Continue to cook and stir over medium-high heat for about 5 minutes, until the sauce has reduced by half. If you don't, the sauce will be runny and not good. Remove from heat, and stir in the butter. Pour over the lamb chops, and serve.