Thaw the Wagyu beef for 24h in the refrigerator.
Thaw the short crust pastry ahead of time by leaving it a room temperature for about 1h.
In a large frying pan, sweat onions and garlic in olive oil over medium-high heat until slightly translucent (about 2 minutes).
Add beef and continue cooking, stirring to crumble meat, for 8-10 minutes or until well done. Add salt and pepper to taste.
Add remaining ingredients and continue cooking over medium heat, stirring often, for about 30 minutes or until potatoes are tender. Let cool.
Taste and adjust seasonings if necessary.
Cover and refrigerate to chill the filling, about 1-2 hours. To cool more quickly, divide into several small containers.
Place rack in bottom of oven and preheat oven to 375F.
Unroll the pastry in a pie plate and press it well into the dish.
Spread the meat mixture on top and cover with the second layer of pastry. Pinch the edges well to seal the pie.
Using a knife, make an incision in the center of the top pastry to allow the cooking steam to escape.
Brush crust with beaten egg.
Bake for 45 minutes to 1 hour or until crust is golden brown.