Heat the olive oil in a saucepan over a medium-low heat.
Add the onions and sweat gently for about 5 minutes or until tender and golden.
Add the garlic, cumin, paprika powder, carrots, celery and a pinch of salt.
Cook for another 5 minutes, stirring occasionally to prevent burning. Add the balsamic vinegar and wait for it to evaporate.
Add the tinned tomatoes, lentils and 1 litre of boiling water. Bring to the boil and then simmer for 10 minutes.
Taste before serving and adjust the seasoning if necessary.