Power up: Cranberries are an antioxidant star, second only to blueberries in disease-fighting compounds.
Bring cranberries, syrup, and water to a boil in a medium saucepan over medium-high heat. Reduce heat to medium and cook until cranberries just start to pop, about 5 minutes. Drain, reserving 1 tablespoon of the cooking liquid. Let cool completely, about 30 minutes.
Stir together cranberries, onion, cilantro, jalapeño, lime zest, lime juice, salt, cayenne pepper, and reserved 1 tablespoon cooking liquid in a medium bowl. Serve at room temperature or chilled.