Preheat a 4 quart pot over medium heat. Saute the onion in olive oil, along with a dash of salt, for about 5 minutes. Add the garlic, thyme, celery seed, pepper and salt and saute a minute more.
Add the lentils, baby carrots, tomatoes and vegetable broth. Cover pot and bring to a boil. Stir occasionally for about 20 minutes, until lentils are slightly softened (now is a good time to start the polenta.)
Add potatoes and leeks. Lower heat to simmer. Cover and cook for about 20 to 30 more minutes, until potatoes are fork tender and lentils are soft. Let sit for 10 minutes or so to allow the flavors to meld. Serve over polenta, garnished with fresh herbs, if you like.