Preheat the oven to 180°C. Cut off any fat from the turkey breast, then place it, skin side up, in a shallow roasting pan. Brush the oil all over the breast and sprinkle with half of the pepper. Cover loosely with foil and roast for about 45 minutes.
Remove the foil and scatter the garlic cloves around the turkey, coating them with the pan juices. Roast, uncovered, for a further 1 1/4 hours or until a thermometer inserted into the thickest part of the breast reads 77°C.
Remove the skin, then transfer the turkey to a platter. Cover it with foil and keep warm. Spoon off all the fat from the roasting pan, leaving the pan juices. With a spoon, squeeze the garlic cloves from their skins into the pan.
Place the roasting pan over moderate heat. Stir in one-third of the stock and heat until bubbling. Transfer the mixture to a blender or food processor and puree until smooth. Return to the pan and bring to the boil over moderate heat. Stir in the remaining stock and pepper and the lemon juice. Simmer for about 3 minutes. Slice the turkey breast and serve with the garlic sauce.