Cut three or four slits (about 2 inches long, ½ inch deep) into eggplant, coat with balsamic vinegar, and grill (or roast in 400° oven about 20 minutes) until eggplant collapses. Scoop flesh from eggplant, discard skin, and roughly chop the flesh. Place in a medium-size mixing bowl. Add ground walnuts, olive oil, salt and pepper, vinegar, and garlic; mix well and check seasoning. Spoon into a serving dish and top with parsley, feta, and chopped walnuts. Serve with warm pita bread.