In small saucepan heat butter over medium heat. Add onion and garlic; cook and stir about 4 minutes or until onion is tender. Stir in blueberries, wine, and sugar. Bring to boiling; reduce heat. Simmer, uncovered, for 10 minutes. Mash berries lightly with a potato masher. Simmer for 2 to 4 minutes more or until thickened. Remove from heat. Stir in mustard; cool. Chill for up to 2 weeks.