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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 to 15 minutes.
Place a large cast iron pan in the Traeger and allow to preheat for 10 to 15 minutes. Slice onion in half and then into to 1/4 inch half-moons.
Put a teaspoon of olive oil in your preheated pan along with a half 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.
Remove onions from grill and turn temp to high.
Wipe out pan or use cast iron griddle and place in grill to preheat for 10 to 15 minutes.
Take 2 slices of bread and butter the sides that will be touching the pan.
Spread a thin layer of mustard and mayo on the other side of bread.
Add 2 slices of each cheese and then top with the onions.
Place your sandwiches in the preheated pan or on griddle and close lid.
Grill for 8 to 10 minutes per side or until well browned and cheese is completely melted.
Remove from grill, slice in half into triangles and serve immediately. Enjoy!