Preheat the oven to 395 degrees F.
Heat the olive oil and butter in a large saucepot over medium-high heat. Saute the mushrooms, potatoes, carrots, leeks, celery, garlic and onions for about 5 minutes. Add the flour to the vegetables to coat well, and then add the white wine. Continue to cook over medium heat for 3 minutes. Add the stock and cream and stir until smooth. Simmer on low heat until the vegetables are tender, about 15 minutes. Add the rockfish, cook for 5 minutes, and then add the oysters, herbs and lemon juice. Season with salt and pepper.
Divide the mixture into 6 large ovenproof ramekins and cover with the rounds of chilled puff pastry. Brush the tops of the pastry with the beaten egg and bake until golden brown, about 15 minutes.
Recipe courtesy of Chef Lynn Crawford, 2009