Heat the oil in a large saucepan over medium heat. Add the onion, carrots and celery and cook for around 10 minutes, until soft. Add the garlic and cook for another 1 minute.
Add the paprika, oregano, thyme, salt and pepper, mixing the herbs in with the vegetables. Add the tinned tomatoes, lentils and finely chopped walnuts and stir everything together, then pour over the stock and give it another mix.
Increase the heat to high and bring to a boil. Reduce the heat to a simmer and cook for 45 minutes, stirring occasionally, until the lentils are cooked through and tender.
10 minutes before serving, cook the pasta in a separate pan according to package instructions (or to your liking). Serve the Bolognese sauce over pasta with a sprinkle of and pepper, to taste.