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.

In a large pot over medium-high heat, cook onions in butter until they start to soften.

Add wine and salt, continue cooking - stirring often - until nicely brown.

Add sugar and beef stock to pan, stir well, and continue cooking until onions are nicely caramelized.

Remove from heat, stir in fresh thyme, season with salt and pepper to taste.

In a small saucepan, melt butter over medium heat. Add garlic, white wine, and beef broth, once again whisking until smooth. Gently bring mixture up to a simmer.

While wine mixture is heating up, whisk egg yolks and corn starch together in a small bowl or measuring glass. once smooth, whisk in milk - a little at a time - until smooth and well incorporated.

Once the wine mix has come to a simmer, add milk mixture, quickly whisking to fully blend the two. Bring up to a simmer again, whisking constantly. As it heats up, add the Gruyere cheese, stirring to melt and combine.

Once custard has thickened to your liking, remove from heat, season with salt and pepper to taste

Spread hot custard in warm crust, top with hot caramelized onions, serve immediately.

Allow everything to cool to room temperature before spreading custard in crust and topping with onions. Chill until serving.