Put the sweet potatoes in a roasting tin. Combine the oil, spices and salt and pepper to taste in a small bowl, pour over the potatoes and stir well to evenly coat.
Roast in a preheated oven, 220°C (425°F), Gas Mark 7, for 30–35 minutes, stirring halfway through, until golden and tender. Leave to cool for 30 minutes.
Meanwhile, blanch the beans in a saucepan of lightly salted boiling water for 2–3 minutes until just tender. Drain and refresh under cold water. Drain again and pat dry.
Put the beans in a large bowl with the cooled sweet potatoes, spinach leaves and pistachio nuts.
Make the dressing. Mix together the yogurt, garlic, chilli, lemon juice, honey and salt and pepper to taste in a bowl. Whisk in the oil until evenly blended. Pour over the salad, stir well and serve.