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 medium-low and simmer, stirring occasionally, until mixture is reduced and syrupy enough to lightly coat the back of a spoon, 30 to 40 minutes. Glaze will thicken more as it cools.