In a small saucepan, combine 1 cup of the grapefruit juice, the sugar and zest, and cook over medium heat until reduced to a thick syrup.
In a clean saucepan, combine the remaining 1/2 cup of grapefruit juice, the vinegar, and shallots. Simmer until reduced to 3 tablespoons. Add the heavy cream and cook for 30 seconds. Gradually whisk in the butter, 1 tablespoon at a time, adding each piece before the previous one has been completely incorporated. Continue until all the butter is incorporated and the sauce coats the back of a spoon, removing the pan from the heat periodically to prevent the sauce from getting too hot and breaking.
Remove from the heat and strain through a fine mesh strainer into a clean pan. Add the grapefruit gastrique, salt, pepper, and grapefruit segments, and stir gently to combine.
Yield: 1 cup