Step 1 Whisk all of the ingredients together in a small saucepan until the arrowroot is dissolved. Place over medium heat and simmer until slightly thickened, about 7 minutes. The mixture does not need to boil.
Step 2 Remove from heat and let the lemon zest steep for 10 minutes. Strain the mixture through a fine-mesh strainer. Let it cool, uncovered, in the refrigerator. ( You can store it, covered, in the refrigerator for up to one week. Makes about 1 1/2 cups.)