Heat olive oil in a medium pan or dutch oven over medium heat until shimmering. Add the carrot, celery, onion, and salt and stir to combine. Cover and let the vegetables sweat, stirring occasionally, until the onions are soft and translucent, about 5 minutes. Add the lentils, water or broth, and bay leaf and bring to a boil. Reduce heat to low and let simmer, covered, until the lentils begin to fall apart, about 20 minutes. Turn off the heat and stir in the lemon juice. Tase and season with additional salt as needed. Ladle into bowls and top with a swirl of yogurt, olive oil, or gremolata.