In a fry pan saute onions in hot oil at mod. heat until soft. Add mushrooms and cook until all juices evaporate. Put in a large saucepan; add spices, stock and 2 1/2 cups water. Bring to a boil, add lentils and simmer, covered, for 15 minutes or until lentils are barely tender. Add tomatoes and simmer,stirring occasionaly, for 10 minutes. Put 1 cup of mixture( mostly solids) in a food processor and blend. Add sour cream;blend.Stir in lemon juice, salt and pepper; blend. Add this to the stew pot and combine thoroughly. Heat until very hot but not boiling. Serves 6.