Defrost pastry dough per instructions (usually takes anywhere from 1-2 hours)
Finely dice onions, garlic, and mushrooms.
Heat oil in a large pan. Cook onions first until translucent, about 2-3 minutes. Add garlic and cook for another minute then mushrooms until soft, about 3 minutes. Then add salt, pepper, and meat, cooking until meat starts to brown but isn't completely cooked, another 3 minutes. Lastly, turn off the heat and stir in sugar and soy sauce.
When pastry dough is firm but no longer frozen, unroll/unfold and cut dough into ~2-3" squares. Preheat oven to 400 degrees.
Two at a time: brush BOTH squares with egg white around the edges. Add 1-2 tablespoons of meat in the center of one square and place the other square on top. Use fingers or a fork to press edges together. Place on a baking sheet lined with a Silpat mat or parchment paper.
Place entire tray of assembled Pate Chaud in the freezer for 5 -10 minutes to firm up pastry.
Remove Pate Chaud from the freezer, brush top of pastries with egg yolk, and pierce with a fork 3 times per pastry.
Bake for 25 minutes or until golden brown on top.