To prepare the beef filling, place a large cast iron pot or a Dutch oven over high heat and add the olive oil. Add the onions, carrots and garlic. Saute the vegetables, frequently stirring, until nicely caramelized, about 5 minutes. Add the beef and continue sauteing for another 8-10 minutes, until the meat browns nicely.
Add the tomato paste, prunes (optional), beer, bouillon cubes and bay leaves. Bring to gentle simmer, stir and lower heat to medium-low. Cover and simmer for 1 hour, stirring every 20 minutes. Take off the lid and continue simmering for another hour, stirring every 20 minutes, until the meat filling thickens. Remove from heat. Remove the bay leaves. With a fork, press on beef chunks to pull meat fibers apart, and set the filling aside to cool.
To prepare the bechamel sauce, in a medium size pot, melt the butter over medium heat. Add the flour and saute, frequently stirring with a whisk, until the mixture turns lightly golden brown and acquires nutty flavor.
Start adding the milk, about 1 cup at a time, while continuously whisking so that no lumps form in the roux. Once all milk is incorporated, continue cooking the sauce until it thickens and starts to bubble.
Remove the bechamel sauce from heat and add the grated Parmesan cheese, salt, pepper, egg yolks and nutmeg. Whisk until well incorporated. Cover the pot and let cool.
Preheat oven to 400F (see note 1).
Lay the first sheet of puff pastry on a lipped baking sheet (see note 2) lined with parchment paper. Brush the edges of the pastry sheet with the egg yolk wash.
Transfer the bechamel sauce to the center of the puff pastry sheet and spread it, leaving about 1 - 1 1/2 inches from the edge unfilled. Spread the meat filling on top of bechamel sauce, again leaving some space around the edges.
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Cover the pie with the second sheet of puff pastry, pressing down on the edges so the pastry can stick together. Left the edges of the pie and crimp all the way around. Brush with egg yolk wash and make slits (see note 3) to allow steam escape during baking.
Bake for about 45-50 minutes, until the top is deep golden brown.
Remove the kreatopita from oven and allow to cool (see note 4) for about 10 minutes before cutting into pieces and serving.