Bring a small pan of water to the boil. Add the eggs. Lower the heat and simmer gently for 6 mins. Lift the eggs out and pop them in a bowl of cold water to cool.
While the eggs cook, bring a larger pan of water to the boil. Scrub the potatoes and chop them into equal-sized chunks. Add to the boiling water. Simmer for 5 mins.
Trim the French beans. Add them to the potatoes. Simmer for another 5 mins till the beans are tender and the potatoes are cooked through. Drain them.
Measure out 1 tbsp Dijon mustard and 1 tbsp red wine vinegar. Whisk with 2 tbsp olive oil and some salt and pepper to make a mustardy dressing. Put to one side.
Pick any large sprigs out of the watercress. Halve the cherry tomatoes. Arrange the watercress, tomatoes, potatoes and French beans on a platter or two plates. Scatter over the black olives.
Peel and halve the eggs, then add them to the salad. Warm a splash of olive oil in a frying pan. Add the proscuitto. Fry for 2-3 mins till the proscuitto is crisp.
Lift the proscuitto out of the pan. Cut into shards. Pop on top of the salad. Spoon over the dressing and serve straight away.