Lay the salmon as one layer in a shallow microwavable dish, season with salt and pepper and cover with cling film. Stab the cling film several times with the point of a knife and microwave on high for 5-6 minutes until the flesh starts to flake. Set aside to cool a little.
Chop or halve the potatoes (depending on size), then cook in a pan of boiling salted water for 10-15 minutes, until tender. Drain well. Slice the spring onions finely.
Make the dressing. Measure the mayonnaise into a small bowl, then whisk in the vinegar, tarragon, olive oil and sugar. Taste and season.
Heat 1 tbsp of olive oil in a frying pan. Cut each slice of ham into 3, then fry quickly in batches until crisp and slightly darkened. Remove the ham and drain on kitchen paper.
Spread the salad leaves over a large platter. Break the salmon into large chunks and put in a large bowl with the warm potatoes and half of the spring onions and capers. Add the dressing and toss lightly to mix. Pile onto the salad leaves and sprinkle with the remaining spring onions and capers. Scatter over the crisp prosciutto.