Position an oven rack 3 inches below the broiler and set the broiler to high.
Toss the tomatillos, pepper and onion with the grapeseed oil in a large bowl. Transfer them to a half-sheet pan and broil until blistered and slightly charred, about 7 minutes. (You'll need to give the pan a shake every now and then to ensure even cooking.)
Let the charred vegetables cool slightly and then blend them on high with the cilantro and salt until smooth. With the blender still running, add the xanthan gum and blend 10 seconds more.
Store tightly covered in the refrigerator for up to 2 weeks. Always serve at room temperature.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.