(Refrigerate any remaining peppers in a jar with brine to cover for another use.) Cut off and discard the pepper stems. Scoop out the seeds with a small spoon (a measuring teaspoon works well). Rinse the peppers to remove any residual seeds,and set in a colander to drain.
Combine tuna,lemon juice,oil,capers and anchovies (if using) in a medium bowl. Fill each pepper with about 1 teaspoon of the tuna mixture and place them on a serving plate. (Depending on the size of the peppers,you may not fill all 24.) Grind some pepper over the stuffed peppers.
Bring vinegar to a boil in a very small saucepan and simmer until syrupy and reduced to about 2 teaspoons 3-3 1/2 minutes. Drizzle the syrup over the peppers and serve. If you arenot going to serve right away,cover and refrigerate stuffed peppers and balsamic syrup separately for up to 4 hours.