Combine 1 teaspoon garlic, 1/4 teaspoon salt, rosemary, mustard, olive oil, and 2 teaspoons vinegar in a small bowl; stir well.
Place lamb chops in a shallow dish.
Spread rosemary mixture evenly over both sides of chops; let stand at room temperature 10 minutes.
While chops stand, bring 1/2 cup balsamic vinegar and remaining 2 teaspoons garlic to a boil in a small saucepan. Reduce heat, and simmer, uncovered, 5 minutes or until reduced to 1/4 cup. Stir in remaining 1/4 teaspoon salt and pepper; set aside, and keep warm.
Place lamb chops on grill rack; cover and grill 4 minutes on each side or until desired degree of doneness.
Place lamb chops on each of 4 plates, and drizzle with vinegar mixture.
