Preheat the oven to 400°F.
In a large skillet, heat a drizzle of oil over medium-high heat. Sauté the onion, celery and carrot for 4-5 minutes, or until soft. Add the garlic, beef and pork; cook, breaking meat up with a spoon, until the meat is no longer pink. As it cooks, sprinkle with thyme, sage, salt and pepper.
Pour in the stock and potato (or rutabaga) and cook, scraping up any browned bits from the bottom of the pan, until most of the moisture has cooked off. Set aside to cool slightly.
On a lightly floured surface, roll the puff pastry out to a 14-15-inch square and transfer to a parchment-lined sheet, positioning it with half the square on the sheet and the other half hanging over (the over hang will fold over the other half).
Pile the filling onto one half of the pastry, leaving about an inch around the edge, and fold the remaining pastry over it.
Press down around the edges with the tines of a fork to seal. Brush the top with egg and cut a few slits with a knife. Bake for 20-30 minutes, or until the pastry is puffed and golden.