Cook the potatoes whole in their skins in a saucepan of lightly salted, boiling water for about 15 minutes until tender.
Meanwhile, mix together all the ingredients for the dressing and cut the beetroot into thin julienne strips about 3 cm long. Cook the peas for 2–3 minutes in a dish in the microwave or in the saucepan of boiling water with the potatoes until tender, then drain well.
Drain the potatoes and thickly slice, then put them in a large serving bowl with the peas, half the spring onions and the pastrami, loosely folded.
Just before serving, very lightly stir the beetroot into the salad and drizzle with the dressing. Scatter the remaining spring onions and the radishes over the top. Serve warm.