Combine cranberries, blueberries, sugar and 1/2 cup water in a medium saucepan over medium high heat and bring to a simmer. Cook 10 to 15 minutes or until cranberries begin to pop. 

Remove from heat and stir in mustard. 

Let cool completely to thicken and serve chilled or at room temperature. 

Can be made up to 3 days ahead of time.