• Mix the sugar and water in a medium heavy bottom saucepan. Bring to a boil over medium heat, reduce to medium low. Do not stir, and cook until the sugar is melted and the color starts to turn light brown, about 5 to 7 minutes, brushing the sides down with a watered pastry brush as necessary.
• Add in the vinegar, taking care because the vinegar will bubble up vigrously. The caramel may harden but it will dissolve as the the vinegar simmers. Simmer this until the liquid has been reduced by half.
• Add in the berries. Continue to simmer until the sauce becomes fragrant and thickens slightly, about 10 minutes.
• Strain through a fine-mesh sieve, and finish the sauce with a little salt if desired.

Preserving:

• Refrigerate: Ladle the gastrique into bowls or jars. Covered, the sauce will keep for up to 3 weeks.
• Freeze: Ladle into an ice cube tray and cover. Alternately, pour into small containers with lids. The sauce will keep this way for up to 6 monts.