Pre-heat oven to 425 degrees. Peel sweet potato and cut into 1/4 inch slices and place into large bowl. Cut lemon in half and squeeze lemon juice over potatoes. Drizzle 2Tbl of olive oil and crumble rosemary over potatoes. Mix well and let marinaide for 5 minutes.
Place cut chicken breasts in bowl with potatoes and mix well. Place potatoes in bottom of a 13" x 11" baking dish and put chicken on top, skin-side down. Cover with plastic wrap and let marinaide in refridgerator for 4 hours.
Take heads of garlic and remove any loose paper-skin. Remove a sliver off the top of each head and drizzle with remaing olive oil. Place garlic, cut-side down, in-between chicken pieces. bake for 60 to 75 minutes, basting occasionally with juices from bottom of pan.
To serve, remove garlic heads and cut in half. When cool enough to handle, squeeze garlic from heads over chicken pieces.