Pat the turkey breast dry and place in the slow cooker. Separate the skin from the meat and rub some olive on the skin, then rub the rest of the surface with the remaining olive oil.
Place 4 sage leaves under the skin on either side of the breast, then pull the skin back over it. Turn the bird meat side down in the slow cooker.
Stuff the cavity with one half the lemon and the herbs. Squeeze the other lemon half over the bird and pour the white wine into the slow cooker. Sprinkle the salt and pepper over the bird.
Place the lid on top and set the slow cooker to automatic. Cook for 6 to 7 hours. (If your slow cooker does not have an automatic setting, select High for 2 hours, then turn back to Low for the remaining 4-5 hours.) Check the temperature at the 6 hour mark. When the temperature reaches 160° F/71° C, remove the herbs and lemon and transfer the bird to a carving board or large tray. Cover with foil and allow it to rest for 20 to 30 minutes. When the internal temperature is 170° F/77° C, the turkey is ready to slice and serve.
Pour the leftover liquid through the strainer into the gravy separator and allow the oil to rise to the top. Once the oil has separated, pour the liquid into the sauce pan and warm over medium heat. Add the sage, thyme, and parsley and cook for about 5 minutes. Remove the herbs.
Bring to a boil. Mix the cornstarch and cold water until the lumps disappear. Whisk into the liquid as it boils and it should start to thicken. Reduce heat and stir in the Madeira. Cook for about 2 more minutes, then serve.