Preheat the oven to 180°C / 390°F. To make the crispy lentils (if using), tip the lentils onto a baking tray and drizzle with olive oil; place the tray in the oven and cook for 20–25 minutes until crispy. Set aside to cool.
Warm the olive oil in a large saucepan set over medium heat; add the garlic and cook for 3–4 minutes until golden. Add the leeks, bay leaf and a pinch of salt; cook for 10 minutes until softened but not coloured.
Stir in the stock, peas, cavolo nero and butter beans; simmer (with the lid on) for 5 minutes until wilted.
Remove the bay leaf and blitz until smooth using a hand-blender or a high-speed blender (you can leave it a little chunkier if you prefer). Season to taste with salt and black pepper.
Divide the soup between bowls and scatter the crispy lentils (if using) over the top.
