FOR Sweet potato:
Peel and dice the red onions and carrot(s). Wash and dice 1 celery stalk. Heat 2 tablespoons of oil in the saucepan and fry the onions, carrots and celery gently for 10 minutes until softened.
Meanwhile, peel 2 garlic cloves and finely chop or crush them. Strip the leaves from 1 stalk of rosemary and finely chop them. Drain and rinse the lentils (see cook’s notes). Wash and drain the spinach.
Add the garlic and rosemary to the saucepan and fry gently for a further 2 minutes. Add the harissa, bay and cumin. Cook for a further 1 minute before adding the lentils and chopped tomatoes. Fill one of the empty tins half full of water.
Add that too and bring to a gentle simmer. Season with salt and pepper. Leave it to simmer while you deal with the sweet potatoes. Wash the sweet potatoes and slice them widthways into 3mm discs. Place the slices in a mixing bowl with 2 tablespoons of oil.
Season generously with salt and pepper. Toss them together so that the slices are all coated and seasoned. Preheat your oven to 200°C/Gas Mark 6. Add the spinach to the lentils. Stir until it wilts. Taste the mix and adjust to your taste with salt and pepper.
Transfer it to the casserole dish. Top the lentils with the sweet potatoes in an overlapping shingle pattern. Place in the oven and bake for 25 minutes until the potatoes are tender and colouring at the edges. Meanwhile, clean the saucepan and fill it with salted water.
Put it on to boil. Wash the spring greens and strip the leaves from their tough central stalks. Lay the leaves on top of each other and roughly shred them. When the bake is ready, boil the greens for 3 minutes, or until tender. Drain and serve beside generous scoops of the bake.