To cook the potatoes: Place the potatoes in a large pan of salted water. Bring to the boil, then cover and simmer for 15–20 minutes, until the potatoes are tender but not breaking up. Drain and allow to cool.
To make the dressing: Whisk oil, lemon juice and mustard together, season to taste.
Begin to build the salad: To a large serving bowl add the fennel and red onion, then gently stir through a tablespoon of the dressing.
To prepare the oranges: Take one of the oranges and cut the ends off, then cut all around the orange to remove the skin and pith. Using the knife gently slice between the segments and the connective membrane. Hold the orange over the serving bowl to catch the juice. Place the segments in the serving bowl with the fennel and red onion and continue until you have made your way all the way around the orange. Repeat on the other orange.
Once the potatoes are cool enough to handle, cut them into slices, place in the serving bowl with the fennel, red onion and oranges. Then add the rocket or watercress and stir through another tablespoon of the dressing. Taste and season.
To serve: Top with chunky pieces of the ham and nuts, and drizzle over more dressing. Bring to the table and let everyone help themselves. Alternatively divide the salad between 4 plates.