Add the balsamic vinegar and allulose (if using) to a small saucepan. Bring to a gentle boil.
Reduce heat to low and simmer for 20 - 25 minutes, or until the volume is reduced to between ½ cup and ⅓ cup (the closer to ⅓ cup the thicker it will be!) and the vinegar coats the back of the spoon, like the picture in the post. It will slightly thicken more as it cools. Be very careful not to simmer the vinegar too high, otherwise, you risk burning it and having to start over!
Store in a jar with a lid in the refrigerator for up to 1 month.
Notes
A note about the carbs: Balsamic glaze (or reduction) is typically not keto, coming in at around 11 net carbs per Tablespoon due to the balsamic vinegar and sweetener. Balsamic vinegar is naturally higher in carbs, coming in at around 3 carbs per tablespoon. Then, the carbs are pretty much doubled once reduced into a glaze. However, we can reduce the carbs by using a sugar-free sweetener, so the only sugar is the natural sugar that comes from the balsamic vinegar itself. When eaten in small amounts as a garnish and made with a keto sugar-free sweetener, such as the allulose used in this recipe, it can be worked into a low-carb diet, coming in at 6 net carbs per Tablespoon, or 3 net carbs per ½ Tablespoon as written in the recipe card.
I don't recommend other sugar-free sweeteners as they may not dissolve well or leave an aftertaste.
Be sure to check out my favorite way to use this recipe: Keto Caprese Salad!