Trim the bones from the lamb chops (or not, as you prefer). Trim any excess fat off the bones, but don't remove any lean or cartilage. Put the bones with the onion, celery, rosemary, marjoram, and peppercorns into a medium saucepan. Cover everything with water and bring to a bare boil. Hold that for one hour. Strain the stock, rinse the saucepan, and reduce the stock to about 1/2 cup.
Sprinkle the lamb noisettes generously with salt and freshly-ground black pepper. In a heavy skillet over medium-high heat with 2 Tbs. of olive oil, sear the chops for about two minutes on each side. Transfer the chops to a pan with a rack, and put them into the 325 degree oven. for about four minutes, until a meat thermometer reads 140 degrees (for a rosy medium).
While that's in the oven, add the remaining olive oil to the skillet. Add the garlic and cook until it begins to brown slightly. Add the balsamic vinegar and bring to a boil.
Add the lamb stock and return to a boil. Dissolve the cornstarch into a teaspoon of water and add it to the sauce. Reduce until the sauce thickens enough to coat a spoon. Remove the garlic cloves.
Season with salt and pepper to taste, then add the mint and parsley. Spoon the sauce over two lamb noisettes per person.