Preheat the oven to 200 degrees C. Place the salmon into a baking dish, season with the tarragon and pour over a tablespoon or two of red wine. Cover with a lid or foil.
Bake the salmon for about 5 minutes, then remove the lid, and add the prawns to the dish. Cover, and return to the oven. Cook for 10 minutes, or until salmon flakes with a fork, and prawns are pink.
Meanwhile bring a large pot of lightly salted water to a boil. Add the spaghetti, and cook for 10 minutes, or until tender. After the first 5 minutes, add the asparagus spears they will be done at the same time.
As you wait for the pasta, heat the oil in a large frying pan over medium-high heat. Add the onion and mushrooms; cook and stir for a few minutes, then add the garlic. Cook until garlic is fragrant, then add the wine. Cook and stir to remove any bits stuck to the pan, then reduce the heat to low and stir in the cream.
Drain the spaghetti and asparagus, and mix into the skillet with the cream sauce. Flake the salmon and mix it in too. Pour onto a serving platter and place the prawns on top. Garnish with grated Parmesan cheese, and serve.