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.