In a large heavy saucepan, heat the oil over moderate heat. Add the onions, garlic and carrots, and sauté for 2 to 3 minutes. Stir in the mint, lentils, stock and water, and bring to a boil.
Adjust the heat so that the mixture simmers gently, cover, and cook for 35 minutes or until the lentils are tender. Stir in the salt and pepper. Serve chunky style with a dollop or yogurt if desired, or purée the soup as directed below.
To purée, drain the mixture in a sieve set over a bowl, reserving the liquid. Place the lentil mixture and ½ cup of the reserved liquid in a blender or food processor and whirl for 1 minute or until puréed. Return the lentils to the saucepan, stir in the remaining liquid, and heat to serving temperature. Garnish each portion with a dollop of yogurt if desired. Makes 6 1¼-cup servings.