Loosen the skin under the turkey breast with a spoon or your fingers, keeping the “pocket intact”. Spoon 2 tbsp of the butter mixture under the skin of each breast. Brush the skin with the extra butter and place on a roasting rack in a roasting pan.
Slice navel oranges into ¼ inch wheels and place under the bottom of each breast on the roasting rack. This will allow the meat to be infused with the citrus as it roasts.
Roast turkey for 1 hour and 40 minutes, basting with a spoon or silicone brush, until the skin is golden brown and the internal temperature of the meat registers at 163F via a digital meat thermometer. Remove it from the oven to rest and as it rests it will come up to 165F, the perfect temperature for a moist turkey.
Remove breasts from pan with turkey forks to a carving board and pour gravy through a fine mesh sieve or cheesecloth into a pitcher, transfer to a gravy boat.
Slice breasts on the diagonal, place on a platter, garnish with fresh thyme and small mandarin oranges if you wish, and serve with the pan gravy.