Douse the lentils in the boiling water, add the salt & let them stand, covered, for an hour. Heat the oil in a pot and add to the vegetable broth powder. Add the diced vegetables, brown sugar, and a bay leaf, and saute for a few minutes before adding it all to the lentils. Cook over a low heat for about 1 1/2 hours. Mix together the flour and the or tomato puree. Add a little of the hot soup, stirring well, then return it all to the pot. Grind in some black pepper, heat through, and serve.