In a small saucepan, combine blueberries, sugar, and vinegar. Heat on medium, stirring and crushing the berries, until it boils for about 5 minutes. Strain the mixture to separate the syrup from the solid berries.
Spread blueberries on a slice of bread, top with cheese, spinach or arugula, more cheese, and fresh ground pepper. Cover with the other slice of bread and toast until golden brown.
Enjoy your Balsamic Blueberry Grilled Cheese Sandwich!
