When ready to cook, start the Traeger grill on Smoke with lid open until fire is established (4 to 5 minutes). Set the temperature to 375 degrees F and preheat, lid closed, for 10 minutes.
Place a large cast iron pan in Traeger and allow to preheat for 10 to 15 minutes.
Slice onion in half and then into to 1/4” inch half-moons. Put 1 Tbsp. of butter and coat bottom of pan. Add onions, sprinkle with a pinch of salt and pepper and close lid.
Cook for 40 minutes stirring every 10 minutes until onions are a rich brown color.
While onions caramelize, form 4-1/4 lb patties with the ground beef. Season both sides with salt and pepper to taste. Remove onions from grill and turn the temperature to High.
Place patties on preheated grill and cook for about 5 minutes per side or until they reach an internal temperature of 160 degrees F.
Wipe out cast iron pan and place back on grill to preheat with the other Tbsp. of butter. Place four slices of bread into skillet then top with a slice of cheese, a patty, then some caramelized onions and another slice of cheese.
Smear a thin layer of mustard on the reaming 4 slices of bread and place mustard side down over the melts. Use a spatula and press down.
After about 2 minutes, carefully flip over and press down to brown the other side. Cook until the cheese is fully melted and the bread is golden brown and crisp on both sides.
Remove from pan and slice in half. Serve immediately with sour pickles. Enjoy!