Add about one to two inches of cold water to a large saucepan, then add the onions, garlic, celery and carrots. Stir them around then season with the freshly ground black pepper. Cover and let them sweat over a low heat for about 10 minutes.
After that, add the stock, red lentils and bouquet garni, bring to simmering point, cover and let the soup simmer very gently for a further 15-20 minutes, stirring occasionally to ensure the ingredients do not stick to the bottom of the pot. You may find that you want to add more hot water to the pot towards the end of the cooking time if it's looking too thick for you.
Remove the bouquet garni before using a stick blender, or if you prefer a traditional blender, to blend the soup to a puree. If you prefer, you can leave the soup as is. It tastes great either way.