• Preheat oven to 350-degrees F. If your oven has a convection roast feature, only set it for 325-degrees F.
• Peel the onions and halve lengthwise. Slice the halves lengthwise into four or five thick slices. Place the onion slices in the center of the baking pan.
• Slice 1 1/2 lemons crosswise into 10-15 slices. (Set the remaining lemon half aside for use later in the recipe.) Place half of the slices on top of the onions. Dot the onions and lemon slices with two tablespoons of butter.
• Rinse the thyme under cool water; pat dry with paper towels. Separate the thyme sprigs into individual stems. Scatter half of the stems over the lemon slices and onions. Set aside the remaining individual thyme stems.
• Remove the neck and giblets from the cavity of the turkey. With a sharp pair of kitchen shears, butterfly the turkey by cutting vertically along each side of the back bone to separate it from the turkey. Discard the backbone or save for another use. Open the turkey and rinse inside and out with cool running water.
• In the sink or on a flat surface, spread the turkey and place it with the inside facing down. Press firmly on the breast bone until you feel/hear a snap. This will help the turkey to remain in a more flattened position in the baking pan.
• Pat the turkey dry with paper towels. Sprinkle the "cavity" side of the turkey with salt and pepper.
• Turn the turkey over, breast side up. Spread turkey and place it cavity side down over the onions, lemon slices, and thyme. Rub the skin with the two remaining tablespoons of softened butter. Sprinkle with salt and pepper.
• Place the remaining lemon slices randomly on top of the turkey. Scatter the remaining single stems of thyme on top of the turkey and lemon slices.
• Bake, uncovered, until a thermometer placed in the thickest part of the turkey thigh, and not resting against bone, reaches an internal temperature of 165-degrees F. This will take approximately 1 hour 45 minutes to 2 hours. It is important to monitor the temperature of the turkey, particularly during the final 30 - 45 minutes of roasting. For a most moist and tender turkey, do not overcook. Remove from oven and allow to sit 10-15 minutes before carving.
• Remove turkey from baking pan and set on a cutting board. Remove lemon slices and thyme sprigs from the turkey and the pan. Reserve onions, if desired, for serving. Cover to keep warm. RESERVE the cooking juices in the pan to make the lemon gravy!

For the lemon sauce or gravy:

• Add enough chicken stock to the reserved juices to make 2 cups. Pour into a 2 quart sauce pot.
• Squeeze the juice from the remaining lemon half and add the juice to the reserved turkey juices. If you have another thyme sprig, toss it into the pot as well. Bring to a boil over medium heat.
• Whisk together some of the chicken stock or water and the corn starch to make a smooth slurry. Add the slurry to the boiling juices, whisking constantly. Allow the gravy to return to a boil. Cook, stirring as needed until the gravy reaches the desired thickness. Thin, if necessary with a little broth or water. Taste. Correct seasonings. Remove thyme sprig before serving.
• Slice the turkey as desired. Serve with lemon gravy on the side.