Preheat the oven to 200°C. Prepare the sauce by heating a small saucepan over a moderate heat until hot. Add the brandy and reduce until there is a teaspoon left, then whisk in the tomato purée and follow with the fish stock.
Bring to a simmer, adjust the seasoning and set to one side to be re-heated later. Roll the pastry out on a lightly floured surface to 5mm thickness. Use a sharp knife to cut out boomerang-shaped pieces of pastry. Brush with the beaten egg, then arrange 4 pieces overlapping one another to create a fan shape. Line a baking tray and carefully lift the pastry fans onto the tray. Bake for 10-12 minutes until golden and risen.
Meanwhile, rub the salmon fillet and tuna with half of the olive oil, then season well. Heat a large, heatproof frying pan over a medium-high heat and start to pan-fry the fish. Turn after 2 minutes, then transfer to oven for 8-10 minutes to finish cooking.
Remove the pastry when it is ready and set to one side. Heat the remaining olive oil in a frying pan over a moderate heat and sauté the prawns for 4-5 minutes until pink, tender and cooked, adding the tomatoes a couple of minutes into cooking. Season well.
Reheat the sauce and remove the fish from the oven. Cut into chunks and arrange on serving plates and place a few prawns alongside. Place a pastry fan next to the seafood, then arrange the raw asparagus and the tomatoes on the plate. Spoon the sauce on top and scatter a few mushrooms on top of the fish. Sprinkle the chopped dill on top and serve immediately.