Put the nuts and spices in a small, dry frying pan and cook for 1 minute. Transfer to a mortar and crush roughly with a pestle, adding a little salt.
Rub 2 tablespoons oil over the lamb racks, season well and press the nut mixture on to the fatty side of each rack. Transfer to a shallow roasting tin and bake in a preheated oven, 220°C (425°F), Gas Mark 7, for 10 minutes. Arrange the peppers around the lamb and cook for 10–15 minutes more for rare to medium lamb.
Swirl the harissa over the yogurt in a bowl. Slice the lamb racks and serve with the peppers, drizzling over the harissa sauce before serving with couscous.