The apples add sweetness to this dish, and roasting intensifies the flavor, without adding any fat.
Heat oven to 425 degrees. Toss the garlic, potatoes, and apples together with olive oil, salt, and pepper in a large nonstick roasting pan. Cook, stirring mixture occasionally for even browning, until golden brown and tender, 45 minutes to 1 hour.
Transfer the mixture to a medium bowl, and add the milk. Gently smash some of the mixture against the side of the bowl, leaving some whole pieces, until all of the milk is incorporated. Season with salt if needed. Serve immediately, topped with a dollop of low-fat sour cream, if desired.