Remove the neck, giblets and any excess fat from the turkey; discard. Remove and discard the plastic pop-up thermometer. Rinse the turkey under cold water and pat dry with paper towels. Refrigerate overnight, uncovered, to let the turkey dry. (It must be completely dry before frying, or the oil will splatter.)
Fill a turkey fryer with the vegetable oil according to the manufacturer's instructions and heat to 400 degrees F.
Meanwhile, mix the Cajun seasoning and granulated onion in a small bowl. Rub the spice mixture inside the turkey cavity and all over the skin. Make sure the hole at the neck is open at least 2 inches so the oil can flow through the bird.
Place the turkey in the drain basket, neck-end first, and slowly lower the basket into the hot oil until the turkey is completely submerged. Once the oil temperature climbs back up to 350 degrees F, start timing: Fry the turkey until a thermometer inserted into the thickest part of the thigh registers 180 degrees F, about 3 1/2 minutes per pound. (A 12-pound bird will take about 50 minutes from the time you lower it into the oil.)
Carefully remove the basket from the oil and transfer to a paper towel-lined baking sheet to drain. Let the turkey rest at least 30 minutes. Make sure that all of the oil has drained out of the cavity, then transfer to a platter.
Photograph by Kana Okada
Recipe courtesy Cat Cora for Food Network Magazine