Unwrap turkey breast, pat dry inside and out, and sprinkle the cavity with salt and pepper to taste.
Stuff the cavity of the turkey with the onions, quartered lemon, and sage sprigs. Turn the turkey breast, cavity side down, and place into an aluminum, disposable baking pan lined with non-stick aluminum foil.
In a microwave-safe bowl, place the butter, soy sauce, lemon juice, lemon zest, and garlic; heat in your microwave just until butter is melted. Using a brush, brush the lemon mixture over the front and sides of the turkey breast, repeating several time. Reserve some of the mixture to baste while the turkey is grilling.
Preheat barbecue or gas grill. Oil grilling rack, and adjust height to between 4 to 6 inches from heat.
Place turkey, using the Indirect Method (see below), on the grill and close lid. Baste several time with the reserved lemon mixture and the resulting juices while the turkey breast cooks. Roast the turkey breast until the the temperature registers an internal temperature of 165 degrees F. on your meat thermometer (juices will run clear when cut with the tip of a knife). Depending on the weight of your turkey breast, this will take from 1 1/2 to 2 hours. NOTE: I like to remove from the heat a little early, cover with aluminum foil, and the the breast continue to cook while the juices settle. The meat will be tender and juicy.
Indirect Method: To grill by the Indirect Method on a gas grill, preheat the grill with all burners on High. Then adjust the burners on each side of the food to the temperature you desire and turn off the burner(s) directly below the food. For best results, place roasts, poultry, or large cuts of meat on a roasting rack set inside a disposable heavy-gauge foil pan. For longer cooking times, add water to the foil pan to keep drippings from burning.
Slice and serve.
Makes 4 to 6 servings.