Heat half the butter in a thick-bottomed pan, then add the spring onions and cook gently until soft. Add half the mint, together with the peas. Pour over enough water to just cover the peas and place the whole prosciutto slices in a layer on top. Simmer gently for 10 minutes, topping up if the water level drops below the peas.
Put half the peas and prosciutto into a food processor with