Step 1. Make the Crust: In a large bowl, whisk the flour, xanthan gum, baking powder, and salt. Add the chilled butter and flatten each piece between your fingers. Create a well and add the cold water, mixing gently until a dough forms. Press into a disk, wrap, and chill for at least 30 minutes.

Step 2. Shape the Crust: Roll the chilled dough into a 10-inch round and fit it into a greased 9-inch deep-dish pie plate. Trim, tuck, and crimp the edges. Pierce the bottom with a fork and freeze for 10 minutes.

Step 3. Par-Bake the Crust: Preheat oven to 425°F. Line the frozen crust with parchment paper and fill with pie weights. Bake for 15 minutes. Remove the weights and parchment, then reduce oven temperature to 350°F.

Step 4. Cook the Filling: In a sauté pan, melt butter and oil. Cook the onion for a minute, then add the mushrooms, salt, and pepper. Cook for about 10 minutes until browned and liquid has evaporated. Drain any remaining liquid.

Step 5. Whisk the Custard: In a medium bowl, beat the eggs and half and half until smooth and well-combined.

Step 6. Assemble the Quiche: Layer half the cheese, the mushroom mixture, and the remaining cheese in the bottom of the par-baked crust. Pour the egg mixture over the top.

Step 7. Bake the Quiche: Place the quiche in the 350°F oven. Bake for about 35 minutes, until lightly golden and mostly set with a slight jiggle in the center.

Step 8. Cool and Serve: Let the finished gluten free quiche cool on a wire rack for at least 20 minutes to set completely. Slice into wedges and serve warm.