Heat the olive oil in a medium–large saucepan. Add the leek and garlic and cook, stirring, until the leek softens. Add the carrot and celery and stir through for 2–3 minutes. Add the curry powder and cook, stirring, for 2 minutes.
Add the tomatoes to the pan and stir through. Then add the soup mix, water and soy sauce. Bring to the boil, then reduce the heat to low and simmer gently for 15 minutes (until the lentils are tender). Turn off the heat.