In a medium (2- to 3-quart) saucepan or Dutch oven, heat the olive oil over medium heat. Add the carrot, celery, onion, and 1/4 teaspoon salt, stir to combine, then cover and let the vegetables sweat until the onions are soft and translucent, about 5 minutes.

Add the lentils, water or broth, and bay leaf. Bring up to a boil, then turn the heat down to low and let simmer, covered, until the lentils begin to fall apart, about 20 minutes.

Turn off the heat, stir in the lemon juice, and add salt to taste. Ladle into bowls and serve. Leftovers will keep refrigerated for 5 days.

To perk up the soup just before serving, top it with a drizzle of peppery olive oil, a dollop of plain yogurt, a sprinkle of sumac, or a spoonful of gremolata.