Whisk balsamic vinegar and honey together in a saucepan over medium heat. Cook, stirring occasionally, until mixture begins to bubble, 3 to 5 minutes.
Stir garlic, rosemary, and garlic salt into vinegar mixture. Cook and stir until mixture starts to foam. Reduce heat to low and simmer, stirring occasionally, until glaze coats the back of a wooden spoon, 30 to 40 minutes.