Preheat the oven to 200°C, fan 180°C, gas mark 6. Cook the sweet potatoes in the microwave for 10 minutes. Transfer to a baking tray and cook in the oven for 25-30 minutes.
Rub the harissa paste over the lamb steaks. Heat the oil in a frying pan and, over a high heat, fry the lamb steaks for 3 minutes on each side, until cooked through. Remove from the pan and leave to rest.
Meanwhile, bring a pan of water to the boil. Cook the runner beans for 3 minutes, then drain and run under cold water to cool.
In a large bowl, mix together the watercress, cooked runner beans, orange zest and segments, the almonds and radishes.
Divide the butter between the sweet potatoes and serve with the lamb steaks and salad.