Preheat the pellet grill or smoker to 275 degrees F using your choice of wood pellets.
Lay the bacon on a wire cooling rack set on top of a sheet tray. Place the sheet tray of bacon in the heated pellet grill. Set the potatoes directly on the grill grates. Close the lid and smoke the bacon and potatoes.
Remove the bacon when it is cooked to your liking, and transfer to paper towels to remove excess grease. Then chop the cooked bacon and set aside.
Smoke the potatoes for about 2-3 hours, or until cooked through and tender. Remove from the grill and allow the potatoes to cool for about 20-30 minutes.
While potatoes are cooling, prepare the other smoked potato salad ingredients. In a skillet over medium heat, saute the diced carrot and celery just until 50% cooked through, so they are still crunchy (just knock the rawness off). Then, transfer to a large mixing bowl.
In the bowl with the carrots and celery, add mayo, sour cream, yellow mustard, diced onion, green onions, relish, cheddar cheese, horseradish, dill, and chopped bacon. Grate one of the cooked potatoes, and add to the other ingredients in the bowl. Then, mix to combine all ingredients.
Cut the cooled potatoes into bite-sized cubes (we prefer to leave the skins on the potatoes for extra smoke flavor). Add the cut potatoes to the mixing bowl with the other ingredients, and fold until combined.
Refrigerate at least 2 hours or overnight. Garnish with reserved sliced green onions, chopped bacon, and/or celery leaves, and serve.