Fill a turkey fryer or large stockpot with enough oil to fry turkey; leave room for turkey so oil doesn't spill over. Heat oil to 400 degrees F (200 degrees C). Prepare a platter layered with food-safe paper bags.

Rinse turkey and thoroughly pat dry with paper towels. Rub seasoning inside the cavity and all over the outer surfaces. Ensure the neck hole is open at least 2 inches to allow oil flow.

Turn off the heat source. Place whole onion into the drain basket and then the turkey, neck-end first. Slowly lower the basket into the fryer until the turkey is fully submerged. Turn heat back on and cook while maintaining oil temperature at 350 degrees F (175 degrees C) for 45 minutes or 3 1/2 minutes per pound.

Carefully remove the basket from the fryer and drain turkey. Insert an instant-read thermometer into the thickest part of the thigh, near the bone, ensuring it reads at least 165 degrees F (74 degrees C) but no more than 180 degrees F (80 degrees C). Transfer turkey to the prepared platter to drain for 10-15 minutes.