Heat the olive oil in a large pot over medium heat. Add the chopped onion and cook until it starts to soften.
Add the celery, carrots, garlic, cumin, oregano, paprika, salt, and pepper. Stir and cook for a few minutes until fragrant.
Add the diced tomatoes and cook briefly until the mixture thickens slightly.
Pour in the vegetable stock and bring to a gentle simmer.
Add the rinsed lentils and stir well.
Cover partially and simmer for 35 to 45 minutes, stirring occasionally, until the lentils and vegetables are tender.
Stir in the chopped parsley or coriander and the lemon juice.
Taste, adjust seasoning, and serve hot.