Preheat the oven to 395 degrees F. Heat 3 tbsp olive oil and 1 tbsp unsalted butter in a large saucepot over medium-high heat. Saute 1/2 cup button mushrooms, quartered, 1/2 cup med-diced potatoes, 1/4 cup med-diced carrots, 1/4 cup med-diced leeks, 3 stalks celery, med-diced, 2 cloves garlic, finely chopped and 1 small onion, med-diced for about 5 min. Add the 1/4 cup all-purpose flour to the vegetables to coat well, and then add 1 cup white wine. Continue to cook over medium heat for 3 minutes. Add 3 cups fish stock and 1 cup heavy cream and stir until smooth. Simmer on low heat until the vegetables are tender, about 15 minutes. Add 1 pound rockfish, skinned and cut into 1-inch cubes, cook for 5 minutes, and then add 2 doz shucked oysters, 1 tbsp chopped fresh chives, 1 tbsp chopped fresh parsley, 1 tbsp chopped fresh thyme and 1 lemon juiced. Season with salt and pepper. Divide the mixture into 6 large ovenproof ramekins and cover with 6 large puff pastry rounds, chilled (1/8-inch thick and 4 inches round). Brush the tops of the pastry with a beaten egg and bake until golden brown, about 15 minutes.