Measure flours, corn starch, and xanthan gum into the bowl of your food processor, blitz to combine.

Add cream cheese, butter, and egg, 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.

In food processor (or by hand, ew) chop mushrooms, onion, and garlic together, until mushrooms and onions are small, fairly uniform sized bits.

Place mushroom mixture in frying pan with olive oil, and cook down until mushrooms are very soft – almost mushy. Add thyme and

sea salt, remove from heat, cool to room temperature.

Once mushroom mixture has cooled, strain off any excess liquid and stir in the sour cream. Chill until use.

Preheat oven to 450F (230 C). Line 2 baking sheets with parchment paper.

Roll chilled dough out to about ¼" thick. Use a large glass or cookie cutter (about 3-4" in diameter) to cut out rounds.

Put a rounded teaspoon or two of filling in the center of each round.

Whisk the egg and water in a mug, brush a small amount along ½ the edge of each round.

Using the egg mix like a glue, fold each round in half to enclose the mushroom filling. Press firmly around the edges to seal completely, arrange turnovers onto a prepared baking sheet.

Brush the tops of turnovers with egg wash, then pierce each with a fork a few times.

Bake for 12-15 minutes, until golden.

Serve hot or cold.