Simmer syrup ingredients in a 1- to 1 1/2-quart nonreactive saucepan over moderate heat until just syrupy and reduced to about 1/4 cup, about 8 minutes. Pour through a sieve into a small bowl, discarding rosemary and peppercorns.
Cut stems and center ribs from chard, discarding any tough portions, then cut stems and ribs crosswise into 1/4-inch-thick slices. Stack chard leaves and roll into cylinders. Cut cylinders crosswise to make 1-inch-wide strips.
Cook onion and garlic in oil in a 12-inch nonstick skillet over moderate heat, stirring occasionally, until onion begins to soften, about 4 minutes. Add chard stems and ribs, salt, and pepper and cook, stirring occasionally, until stems are just tender, about 6 minutes. Stir in chard leaves and water and cook, stirring occasionally, until tender, about 8 minutes.
Broil chops while chard cooks:.
Preheat broiler. Sprinkle chops with garlic, salt, rosemary, and pepper, then broil on a lightly oiled broiler pan, 4 to 5 inches from heat, turning over once, for medium-rare, 6 to 7 minutes total. Serve chops and chard drizzled with balsamic syrup.