Start by adding sugar and a small amount of water to a medium, heavy-bottomed saucepan. Add only enough water until the mixture resembles wet sand. (In this case I used 1/2 cup sugar and 2 tablespoons water.) The tip of a spoon pulled through the mixture should leave a trail.
Heat the sugar mixture over medium-high heat until the sugar granules completely dissolve and bubbles form.
Watch as the sugar begins to caramelize. You'll see lighter shades of brown first—stop at this stage if you plan on making a less intense gastrique to pair with, say, a delicate meat or fish dish. (The darker the caramelization, the deeper the flavor.) For a more concentrated taste, continue to let the sugar caramelize to a deep, golden color. Swirl the pan gently to help it cook evenly, but don't agitate too much, or your caramel may crystallize on you!
Safety warning: Do not touch the sugar or place any non-heat-proof utensils in it at this time, as it is extremely hot.
When you've reached the desired level of caramelization, add vinegar (in equal proportion to the sugar). This step can be intimidating, as the sugar mixture is quite hot! To cool it quickly and keep the splattering to a minimum, pour in the vinegar swiftly rather than adding it little by little.
The sugar will immediately harden, creating an odd, layered-looking design in the pan as shown.
Continue to cook until the sugar re-dissolves, taking the mixture back to a liquid state.
At this point, flavor away! Add spices, seasonings, fresh foods or liquids such as alcohol, juice or pan juices and adjust to taste. Keep in mind, if you add liquid flavorings, you may need to reduce it further to form a thickened, syrupy consistency.
Finish adding flavorings like berries, chopped fruits, seasonings and herbs. Cook to incorporate.
Serve warm, or reserve and reheat just before serving.