Boil the lentils in about 1 litre of water. Cover and cook over low heat for about 10 minutes. Peel onion and garlic, dice finely. Clean, wash and thinly slice the celery.
Drain the lentils while collecting the cooking water. Carve tomatoes crosswise, blanch with boiling water and rinse with cold water. Skin tomatoes and cut into cubes. Cut the sausages into slices.
Heat the oil and fry the sausage slices in it. Add onion, garlic and celery and fry briefly. Add tomatoes and oregano and simmer at low heat for about 5 minutes. Add 350 ml cooking water and broth, cover and simmer on low heat for about 15 minutes.
Add the lentils, heat and season to taste with salt and pepper. Serve garnished with oregano.