Heat a large pot over medium heat and add the oil. When hot, add the meat if using and brown for 3 to 5 minutes. Add the onions, celery, carrots, garlic and spice and cook, stirring occasionally, until starting to soften, 2 to 3 minutes.
Add 6 cups water and scrape up any brown bits from the bottom of the pot. Add the lentils and herb and bring to a simmer. Add the potatoes and simmer until the lentils and vegetables are tender, about 45 minutes. Add a splash of vinegar to taste and serve the stew in bowls. A fried egg on top would be delightful.
For more ingredient substitution ideas, see the article below.