Season all sides of the porterhouse steak with Dad's All Purpose rub and Dad's Steak seasoning.
Set aside in room temperature for 45 minutes to 1 hour.
Preheat the grill to a high heat or 400 degrees. Set it for dual zone cooking.
Place the seasoned porterhouse steak on the indirect side first. Cook until the internal temperature reads 110 degrees. Flip it and rotate every 3-5 minutes to ensure an even cook on both cuts.
Next, move it to the direct side and sear all sides of the steak not just the top and bottom. For a perfect medium rare, remove it off the grill between 120 and 125 degrees.
Let it rest for 10-15 minutes.
For the horseradish sauce, in a mixing bowl, add freshly grounded horseradish, sour cream, mayo, a splash of apple cider vinegar and chopped chives. Whisk the ingredients together until the liquid has a smooth consistency.