Heat the oven to 200°C (400°F, gas mark 6). In a bowl, stir together the oil and ½ teaspoon salt, then use your hands to turn and coat the potatoes in the mixture. Place the potatoes on a baking tray and roast for 50-55 minutes until soft when squeezed and crisp on the outside.
Meanwhile, to make the dressing, place the cumin in a small ovenproof dish, such as a ramekin, and toast it in the hot oven for no more than 2 minutes while the potatoes are roasting. Do not let it burn. Remove from the oven and leave it to cool.
In a small bowl, stir together the cooled cumin, lemon juice and yoghurt and add salt and pepper to taste. Cover and chill, or set aside at a cool room temperature.
Arrange the salad leaves on four serving plates. When the potatoes are done, remove them from the oven and, using an oven mitt for protection, quarter them lengthways into wedges.
Put five potato wedges on each plate. Drizzle the warm potatoes with the dressing and serve.
Rubbing oil into the potatoes and baking them on a tray instead of in a dish of oil produces crisp roast potatoes with almost no fat.