Combine the clementines, zest, grapefruit, sugar, and a pinch of salt in a medium pot over low heat.
Cook, stirring occasionally, until the juices are thick, 45 to 55 minutes.
Stir in the lemon juice. Transfer to a glass jar and refrigerate for up to 3 weeks.
Tip: A knife—not your fingers—is the quickest way to peel a grapefruit after zesting. It also does the best job of removing the large amount of bitter white pith.