In a large pot, melt the butter over medium-low heat. Stir in the onions and cook until wilted and translucent. Sprinkle the flour over the onions, and let it sit for a moment, until the flour is soaked into the melted butter. Slowly whisk in the flour, then gradually add the chicken stock, continually whisking. As you stir, the sauce will thicken. Whisk in the half and half, then season to taste with salt and pepper.
Stir in the turkey, pumpkin, peas, mushrooms, and potatoes. Season again with a little more salt and pepper. Remove from heat, divide the filling between individual ramekins or oven-proof bowls, and let it sit on a lined baking tray while you prepare the pastry.
On a lightly floured surface, lay the puff pastry and cut circles of pastry about 2 cm larger in diameter than your ramekins. If you like, you can also cut leaf shapes out of the pastry and use that instead of, or in addition to, the pastry rounds.
Cover the ramekins with pastry rounds and decoration, and cut slits in the pastry to let it vent while baking.
Whisk together the egg and tablespoon of water. Lightly brush the egg wash onto the puff pastry and season with salt.
Place the baking tray with the ramekins in the oven and bake for 30-35 minutes, or until the puff pastry is golden brown and has risen, and the filling is bubbling within. If you find that the puff pastry is deflating upon removing from the oven, you may need to bake it for a little longer.
Let cool slightly before serving.
The number of servings will vary depending on the size of your ramekins.