Measure cheese, flours, starches, xanthan gum, salt, and baking powder into the bowl of your food processor, blitz to combine.
Add egg and lard to the bowl, blitz a few times until mixture resembles gravel.
Stream in cold water as you run the food processor, just long enough to bring it together as a dough.
Remove dough from processor, knead lightly to bring it together as a ball. Wrap in plastic film, chill for 1 hour.
Preheat oven to 350 F (180 C).
Generously dust work surface with corn starch, Roll chilled dough out to a rough circle that's slightly bigger than 12" in diameter.
Carefully transfer rolled dough to tart pan, smoothing the bottom and pressing the dough into the sides of the pan. Trim any excess dough that extends past the top of the pan, prick the flat (bottom) surface of the crust a few times with a fork.
Bake for 25-30 minutes or until golden brown. While tart is baking, prepare your caramelized onions.