Roasted Balsalmic Chicken with Horsy sauce

8 boneless skinless chicken breast halves
salt and pepper
1/3 c balsamic vinegar
2 Tbsp Dijon mustard
4 cups rice
1 cup milk
1/2 tsp garlic powder
1/2 cup sour cream
2 Tbsp prepared horseradish

Preheat overn to 400 degrees.  Coat large baking sheet with cooking spray.  Season chicken all over with salt and pepper.  Arrange on baking sheet.  In a small bowl whisk together balsamic vinegar and mustard.  Brush mixture all over chicken.  Roast chicken for 25 min.  Cook rice according to package directions.  In a small saucepan over medium heat, combine milk and garlic powder.  Scald milk by heating until tiny bubbles appear around the edges of the pan.  Transfer mixture to a blender and add sour cream and horseradish.  Process until blended and foamy.  Serve chicken with horseradish over top.