Preheat oven to 400 degrees F. Coat a 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.
Arrange acorn squash on a separate baking sheet, flesh side up, and season the flesh with salt and pepper. Arrange pumpkin seeds on sheet next to squash. Arrange potatoes on the baking sheet with the chicken or the baking sheet with the squash (or a third baking sheet--wherever they fit!) and spray with cooking spray. Season potatoes with salt and black pepper.
Roast chicken, squash and potatoes for 25 minutes, until chicken is cooked through and squash and potatoes are tender, shaking the pan with the pumpkin seeds occasionally to prevent burning.
Cook rice according to package directions.
In a small saucepan over medium heat, combine milk and garlic powder. Scald milk by heating just until tiny bubbles appear around the edges of pan. Transfer mixture to a blender and add sour cream and horseradish. Process until blended and foamy.
Serve half of the chicken with horseradish foam spooned over top (reserve remaining chicken for another meal). Serve squash with pumpkin seeds sprinkled over top. Serve half of the rice with this meal and reserve the other half for another meal. Reserve potatoes for another meal.