Combine the first four ingredients in a blender, or use a handheld immersion blender. Process until smooth, stopping to scrape down the sides.
Remove the giblets, and rinse the chicken with cold water; pat dry. Place the sage leaf and 1 garlic clove in the cavity of the chicken. Tie the legs together with string. Lift the wingtips up and over back, and tuck under the bird to keep them from burning. Place the chicken on a lightly greased rack in a shallow roasting pan, breast side up. Insert a meat thermometer into the meaty portion of a thigh, making sure it doesn't touch bone.
Brush the chicken with the oil mixture.Rub some of the oil mixture under the skin of the chicken so the flavor penetrates the meat. Bake uncovered, at 500 degrees for 10 minutes. Reduce the oven temperature to 350 degrees, and bake an additional 50 minutes or until the thermometer registers 180 degrees. Remove the sage and garlic from the chicken cavities; set aside. Remove the chickens to a serving platter and set aside. Keep them warm. Skim the fat from the drippings; reserve the pan drippings.
Combine the dried sage flakes, 1 garlic clove, vinegar, and next 4 ingredients and blend with a handheld immersion blender (or use regular blender); process until smooth. Add the reserved pan drippings; process until smooth, stopping once to scrape down the sides. Serve the chicken with the sauce.