Heat the oil in a large pot over a low heat, then add the onion, carrots and potato and cook, stirring often, for 7 minutes or until softened but not browned.
Stir in the garlic, tomato puree and harissa paste and cook for a further minute.
Stir in the vegetable stock, water, canned tomatoes, and lentils, bring to the boil then reduce the heat and simmer gently for 20 minutes or until the vegetables and lentils are tender and cooked. Season with the salt and pepper to taste (you may not need to add salt if the stock is salty).
Divide the soup into bowls and serve topped with a dollop of yogurt and chopped cilantro.
Notes
Rinse lentils well to remove excess starch.
Use a good quality vegetable stock for deeper flavor.
Adjust spice levels to taste—harissa can be powerful!