Vegetarian lentil cottage pie

Ingredients

• 1 cauliflower head, cut into florets
• 240g/9oz green beans, halved
• 1 tbsp vegetable oil
• salt
• freshly ground black pepper
• 4 celery sticks, finely chopped
• 250g/9oz swede, peeled and grated
• 8 carrots, peeled and grated
• 1 tbsp dried mixed herbs
• 3 tbsp tomato purée
• 3 x 400g tins green lentils, drained and rinsed
• 1 reduced salt vegetable stock cube
• 500ml/18fl oz boiling water
• pinches freshly grated nutmeg
• 600g/1lb 5oz white potatoes, scrubbed and roughly chopped
• 30g/1oz butter