Trim any silver skin or large pieces of fat from the roast. Also, trim off any thin pieces of meat from the end of the roast (they will burn) to get a nice uniform shape.

Coat the roast on all sides with a thin layer of mustard.

Combine the garlic powder, onion powder, paprika, salt, pepper, and celery salt in a small mixing bowl. Coat the roast on all sides with a layer of the spice mixture. The layer of the spice mixture should be just thick enough so you can no longer see the mustard.

Create a two zone fire in your grill. The goal of this is to create one area of your grill that is meant for high, direct heat similar to a stove, and a second area with lower, indirect heat similar to your oven. This is done by banking a small amount of coals on one side of your grill. I would recommend using fewer coals than you think you need. You can always add more coals to increase the heat, but it is a lot harder to remove lit coals to cool down your grill. If you have, throw on some wood chips or chunks for additional smoke flavor. The wood is not a must, but it is will give your meat a little something extra.

Put the roast on the cooler, indirect heat side of the grill (the side without the coals). Put the probe from your thermometer in the middle of the thickest part of the roast. Cover the grill and allow the roast to cook until it hits 130°F.

Once the roast hits 130°F remove the thermometer and move it to the hotter, direct heat side of the grill and cook on each side until you get a nice char. This should get the internal temperature to about 145°F, medium rare. Remove the roast from the grill to a cutting board and cover loosely with foil, allowing it to rest for at least ten minutes

While the roast rests, throw the onions on the hot side of the grill and cook until they start to char. Keep an eye on them as they will burn easily and be careful to not let them slip through the gaps in the grill grate.

Peel the horseradish and add it along with the mayo, garlic clove, lemon juice, mustard, salt and pepper to the blender or food processor. Blend until smooth. This can be done in advance, but not too early or the horseradish will start losing its bite.

Once the roast has rested, slice it as thin as you can against the grain. This will shorten the fibers of the meat creating a more tender tasting piece of meat.

Toast the buns on the hot side of the grill, being careful not to let them burn. Put some of the horseradish sauce on the bottom of the bun, then the pickles, followed by the sliced beef. Top with the grilled onions and the top half of the bun. Serve immediately.