Heat 1 tablespoon of the oil in a large frying pan, add the prosciutto and cook over a high heat for 2 minutes until crisp and golden, turning once. Remove from the pan and place in a large, heatproof salad bowl.
Add the chicken livers and walnuts to the pan and cook over a high heat for 3–4 minutes until golden and the chicken livers are cooked through, but still slightly pink in the centre. Add to the prosciutto and toss in the watercress and rocket.
In a jug, whisk together the vinegar and remaining oil, then pour over the salad and toss well to lightly coat. Serve warm with crusty bread, if liked.