Place lentils, water and salt in a saucepan. Bring to a boil, lower heat to the slowest possible simmer and cook quietly, partially covered, for 20-30 minutes. At this point, I sometimes feel that the 'broth' has become quite thick because of the lentils, so I add another 1-2 cups of water or broth so it looks like the right consistency.
Add vegetables (except tomatoes), herbs, and black pepper. Partially cover, and let simmer for another 20-30 minutes, stirring occasionally.
Add tomatoes and let soup cook for at least 5 minutes more.