Bring a large saucepan of lightly salted water to the boil and cook the tagliatelle for 8–10 minutes until just tender, then drain.
Meanwhile, heat the oil in a large, heavy-based frying pan and cook the onion over a medium heat, stirring occasionally, for 5 minutes until softened. Add the spring onions and garlic and cook, stirring frequently, for 2 minutes. Add the prawns and cook over a high heat, stirring, for 2 minutes, then add the spinach leaves and cook, stirring constantly, for 1–2 minutes until the spinach has wilted and the prawns have turned pink.
Add the mascarpone and stir through until melted and hot. Season with plenty of pepper, then add the Parmesan and drained pasta. Heat through, tossing, for 1–2 minutes until piping hot.
Serve in warmed serving bowls with warm crusty wholemeal bread, if liked.