Leave the turkey in the refrigerator to thaw completely. Drain and wipe with kitchen paper. You put it on a tray.
Cut the butter into thin slices (3-4 mm thick) and place them under the skin of the breast. Be careful not to tear the skin.
Season the turkey belly with salt and pepper and add the liver and heart. Close the opening with half an orange and tie the legs with kitchen twine.
In a large pan, put the olive oil, onions, celery, carrots and thyme sprigs. Cut half and whole orange into slices. Place them on top of the vegetables and place the turkey on top. Season the turkey with salt and pepper on all sides.
Drizzle the dressing around the turkey. Cover the pan tightly, first with non-stick paper and then with a double layer of silver foil. Bake at 200 C in the air for 30 minutes. Lower the oven to 175 C and continue baking for another 2 hours. Uncover the turkey and add the chestnuts to the pan. Continue baking for another half hour until the turkey is golden brown.
Place the turkey on a cutting board or platter and cover with foil. You let it stand for an hour before cutting it.
To make the gravy, remove the chestnuts in a bowl. Pass the pan juices through a strainer. Put the butter in a small saucepan to melt. Add the flour and mix until it smells and takes on a little color. Add the pan juices and mix until you have a slightly thicker sauce.
Serve the turkey with the chestnuts and sauce on the side.
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