Heat the sesame seeds in a large dry saucepan until lightly toasted. Tip out into a small bowl.
Add the oil to the pan and gently fry the onion and carrots for 10 minutes until softened. Add the stock and thyme and bring to the boil. Reduce the heat, cover and cook very gently for 10 minutes.
Tip in the lentils, cover and cook gently for a further 20 minutes or until the lentils are soft. Remove from the heat, leave to stand for 5 minutes and then and stir in the tahini paste. Season to taste with salt and pepper.
Ladle into bowls and top with spoonfuls of crème fraîche or Greek yogurt. Serve sprinkled with extra sesame seeds and thyme.