Do not let the fancy name fool you. Gastriques are really nothing more than the western world version of sweet and sour. Using equal parts vinegar and sugar, simmering them down to half volume, and voila, you have a basic gastrique.
Using a vegetable peeler skin one half of an orange, getting the colored part of the peel while leaving the white pith behind. In a small pot combine ½ cup sugar, ½ cup apple cider vinegar and the rinds.
Bring to a soft boil and continue simmering for approximately 15 minutes until it has become sticky with a syrup consistency.
Remove the orange rind.
Put a thin spread on the plate under your scallops, or drizzle lightly over the top.