Heat the oil and butter in a medium saucepan over a low heat and fry the onion for 10 mins, or until softened and translucent. Add the garlic and cook for 2 mins more. Add the cream, mascarpone, parmesan and stock cube. Give it a stir and add the cooked salmon to heat through.
Meanwhile, cook the pasta following pack instructions. Reserve 100ml of the pasta water. Toss the pasta in the creamy sauce and enough of the reserved water to loosen. Season with black pepper. Top with the parsley and serve with a green salad.