Lay the prawns on a plate and season generously with salt.
Heat a splash of olive oil in a large frying pan over high heat. Add the prawns and cook for 30–60 seconds on each side until lightly coloured but not quite cooked through. Transfer back to the plate.
Reduce the heat to medium and add another splash of olive oil if needed. Cook the shallot, the white part of the spring onion and the garlic for 2–3 minutes until softened and fragrant. Stir through the Dijon mustard.
Pour in the white wine and allow it to reduce by about half, scraping up any caramelised bits from the bottom of the pan.
Add the cream and parsley, then simmer gently for 5–10 minutes until the sauce has reduced by about half and coats the back of a spoon.
Return the prawns to the pan, add the Parmigiano Reggiano and squeeze in the lemon juice. Toss everything together for 1–2 minutes, just until the prawns are cooked through. Be careful not to overcook them—they only need a minute or two in the sauce.
Serve over steamed rice, scatter over the green spring onion and a little extra parsley, and finish with a wedge of lemon.
