Stir together garlic and rosemary. Set aside 2 teaspoons.
Place potato pieces in a lightly greased 13- x 9-inch pan. Coat potato with vegetable cooking spray; toss with remaining garlic mixture.
Loosen skin from chicken breasts by inserting fingers and gently pushing between the skin and meat. Place 1/2 teaspoon reserved garlic mixture beneath skin of each breast half. Place chicken, skin side up, on potato. Coat chicken with cooking spray; sprinkle with salt and pepper.
Bake at 425° for 45 minutes or until done.