In a very large stockpot or special deep fryer, heat oil to 200 degrees C. Be sure to leave room for the turkey, or oil will spill over. Layer a large platter with a single layer of flattened food-safe paper bags.
Pat turkey dry with paper towels. Rub seasoning over turkey inside and out. Make sure the hole at neck end is open at least 5cm so the oil can flow freely through the bird.
Place whole onion and turkey in drain basket. The turkey should be placed in the basket neck-end first. Slowly lower basket into hot oil to completely cover turkey. Maintain the temperature of the oil at 175 degrees C, and cook turkey for 7 minutes per kilogram, about 45 minutes.
Carefully remove basket from oil and drain turkey. Insert a meat thermometer into the thickest part of the thigh; the internal temperature must be 80 degrees C. Finish draining turkey on the prepared platter.