Stabilize your grill over direct heat at 500°F and bring a roasting tray or skillet to temperature.
Once the grill has stabilized and the pan is hot, add the oil and quickly follow with the quartered mushrooms. Close the dome and roast for 4 to 5 minutes, shaking occasionally and roast/sear for another 5 minutes. You should see good color and much of the water of the mushrooms evaporate.
Once you like the caramelization and color on the mushrooms, de-glaze with the sherry (be careful of flame). Next, add the garlic and whip in to fully incorporate the flavors.
Follow with the low sodium soy sauce and fresh black pepper. Close the dome and allow the liquid to cook and reduce by half, about 3 to 5 minutes.
Mount the butter (swirl it into the cooking liquid and allow it to glaze the mushrooms). Drop the scallions into the simmering liquid. Allow to soften for 1 minute and transfer the mushrooms and sauce onto a presentation plate. Enjoy!