Preheat your Traeger Flatrock to medium high in one zone.
Wash the peppers and rub all over in oil.
Slice the onion into thick rounds and rub with oil.
Place the oiled peppers and onions on the hot griddle and cook until the pepper skins are blackened, about 15 minutes.
Use a burger press to hold the peppers flat on the griddle if needed.
Remove the charred peppers and onions from the griddle and place them in a plastic bag.
Seal and let sit for 15 minutes.
Once the peppers and onions have steamed, carefully remove the charred skin.
Chop the peppers and onions (removing the seeds) until diced finely, then set aside.
Preheat the Traeger Flatrock to medium heat.
Portion the beef into 1/4 pound balls.
On the griddle, place a heaping spoonful of the pepper/onion mix and a little bit of oil in four spots.
Saute the peppers until beginning to turn golden brown.
Place a ball of ground beef on each pile of peppers.
Put a square of parchment paper on top of a hamburger ball.
Using the burger press, push down firmly until the meat is very thin and incorporated with the peppers.
Repeat with remaining balls.
Season the meat with the Salt Pepper Garlic blend and allow to cook for 2-3 minutes.
Meanwhile, combine the garlic, mayo and pepper and stir until well combined.
Flip the burger patties on the griddle.
Top each patty with a scoop of pub cheese.
Cover the burgers with a steam dome.
Briefly toast the buns.
Assemble the burgers by spreading some garlic mayo on the toasted bun, topped with two burger patties.
Add any remaining peppers from the griddle and then add the top bun.
Slice in half and serve your green chile smash burgers immediately.