Prepare the mushrooms. Clean any stems with dirt or debris, and discard. Put mushrooms in a deep bowl, fill with cold water, and gently agitate mushrooms, allowing dirt to fall to the bottom of the bowl. Lift mushrooms out of the bowl and place in a colander (this leaves all the dirt at the bottom). In small handfuls, quickly rinse a second time, under gently running water, and place mushrooms on a kitchen towel.
Cut larger mushrooms in half, lengthwise, and gently pat with a towel to remove excess water.
Heat a 10 inch cast iron skillet on high heat. Add the mushrooms to the dry pan. Gently stir with a spatula, allowing the water to quickly escape.* Keep stirring gently until all water is cooked off, 5-7 min.
Turn heat down to medium. Add 2 big glubs of avocado oil to the mushrooms, and the minced garlic. Stir to coat and to keep the garlic from burning, another minute or two of cooking, then remove the mushroom/garlic mixture into the bowl of a food processor fitted with a blade.
Process the mushrooms until resembling the texture of ground meat, 3-5 quick pulses, 1 second long each.
Spread the ground meat loosely into a shallow, wide bowl; add the mushroom/garlic mixture, and egg. Season with salt, pepper, and a pinch or two of sage. Fold the mixture into the meat until well incorporated.**
Form the mixture into 4 even sized balls, and shape into tall patties, depressing a hole into the center for even cooking.
Turn Traeger on to Smoke setting. Smoke burgers for 40 minutes.
Add the scallions to the grill and remove the burgers. Turn temp up to 350 F. When grill come to temperature, put burgers back on, cooking an additional 5 min per side (10 more minutes total). Oil the buns, and place on grill, turning over after a couple of minutes.
Remove the burgers, and tent with foil. Remove the scallions, and chop finely, including all the charred bits. Mix the softened butter with the scallions in a medium bowl.
Smear the scallion butter onto toasted buns, add the smoked mushroom burgers, and your favorite toppings.