Blot excess moisture from the tofu, then transfer to the bowl of a food processor or the jar of a blender. Add the salt, garlic, mustard, and lemon juice (or vinegar). Whisk together the xanthan gum and oil, then add to the processor; you don’t have to stream it in like regular mayo. Run the machine for about 30 seconds, or until a thick, creamy mixture forms. You should be able to plop
it from a knife. If it’s runny, blend in 1 or 2 extra pinches of xanthan gum. The mayonnaise firms up and the flavor intensifies during resting. Adjust the flavor as needed with salt, mustard, lemon juice, or vinegar. Transfer to a jar then let sit for an hour before using; refrigerate for up to a week. If it separates slightly, stir it up.