6 onions (about 900g/2lb in total), peeled and finely sliced
Salt and freshly ground black pepper
150g/5oz Puy lentils
3 tsp finely chopped rosemary leaves
1ltr/1¾pt hot chicken or vegetable stock
For the Gruyère croutons
4 slices white bread
75-100g/3-4oz Gruyère cheese, grated
1. Add the oil to a large saucepan over a medium heat. When nearly hot, add the onions. Season with salt and freshly ground black pepper and cook, uncovered, for 25-30 minutes, stirring regularly and scraping the bottom of the pan to dislodge the caramelised pieces, until the onions are a deep golden brown in colour.
2. Add the lentils, rosemary and hot chicken or vegetable stock. Bring to the boil then reduce the heat, cover with a lid and simmer for about 25 minutes, or until the lentils are tender.
3. Shortly before the soup is ready, preheat the grill to high to make the croutons.
4. Toast the bread on both sides then cover each slice with a thick layer of grated cheese. Place back under the grill and toast until the cheese is bubbling and completely melted.
5. Season the finished soup with salt and freshly ground pepper to taste, then divide between 4 warmed serving bowls. Place a whole crouton on top of each bowl of soup and add a grinding of black pepper. (If the slices of bread are too large, cut them into squares before serving.) Serve straight away.