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.
Heat the grill to a medium high setting. Tumble the cherry tomatoes onto a baking tray. Slide under the grill and cook for 5 mins till the skins are beginning to burst open.
While the tomatoes and 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 7 mins.
Trim the French beans. Add them to the potatoes. Simmer for another 3 mins till the beans are just tender and the potatoes are cooked through. Drain them.
Peel and grate the garlic. Juice the lemon. Whisk half of the juice with 1 tbsp Dijon mustard and the garlic. 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. Arrange the watercress, grilled tomatoes, potatoes and French beans on a platter or two plates. Scatter over the black olives. Drizzle over any juices from the tomatoes.
Peel and halve the eggs, then add them to the salad. Warm 1 tbsp olive oil in a frying pan. Add the pancetta. Fry for 2-3 mins till the pancetta is crisp.
Lift the pancetta out of the pan. Cut into shards. Pop on top of the salad. Spoon over the dressing and serve straight away.