Heat oven to 220C/200C fan/gas 7. Rub the oil over the bottom of a large roasting tin or baking dish. Season the salmon with black pepper and a little salt, then wrap a piece of prosciutto around the middle of each fillet. Roast for 10 mins, then toss the asparagus into the oily tin and roast for 5 mins more.
Meanwhile, boil the pasta following pack instructions, adding the peas and beans 2 mins before the end of cooking. Once the vegetables and pasta are just tender, reserve a cup of the cooking water, then drain.
Remove the salmon from the tin, then add the mascarpone, pasta, 3 tbsp of the cooking water, the lemon zest and juice, and tarragon. Season to taste and toss until the pasta is covered with creamy sauce. Serve with the fish and a lemon wedge.