Place pork chops on the unheated rack of a broiler pan. Arrange onions around pork chops. Set pan in oven and adjust oven rack so that top of meat is 4 to 5 inches from broiler element. Remove pan and chops. Preheat broiler. Return pan to the broiler and broil, without turning, for 9 minutes.
Meanwhile, for glaze, in a medium saucepan bring apple juice or vermouth to boiling; boil uncovered on high for 5 to 6 minutes or until reduced to 1/2 cup.
Meanwhile, combine cornstarch with cold water; stir into apple juice or vermouth. Cook and stir until slightly thickened and bubbly. Cook and stir for 2 minutes more. Remove from heat. Stir in oil, garlic, 2 tablespoons of the fresh thyme or 1 teaspoon of the dried thyme, the pepper, and salt. Remove 2 tablespoons of the glaze and set aside. Turn pork; broil 9 to 13 minutes more or until juices run clear. Using a pastry brush, coat pork chops and onions with glaze during the last 5 minutes of cooking.
Remove onions to a small serving bowl. Remove pork chops to serving platter or dish. Stir the reserved glaze and the remaining thyme into onions. Pass with pork chops. Garnish with fresh thyme sprigs, if desired.