Chop the onions, carrots and celery into small chunks. In a large pan, heat the olive oil and sauté the vegetables for around 10 minutes, or until they have softened.
Add the pieces of sweet potato and sauté for one minute. Then add the lentils and pour in the stock. Mix in the tomato puree and the sweet paprika and stir well. Cover and bring to the boil.
When the soup hits boiling point, turn down the heat and simmer, cover, for around 25 minutes. The soup is ready when the sweet potato is soft and the lentils are cooked through.
Blend the soup until smooth and serve with slices of toasted rye bread.
You can replace the sweet potatoes for white potatoes, if you prefer, which tastes just as good. Typically, the dish won't need any seasoning, but you can add salt and pepper to taste.