Position an oven rack 3 inches below the broiler and set the broiler to high.
Toss the tomatillos, chile, 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.