- 2 tablespoons olive oil
 - 1 cup chopped onion
 - 3/4 cup diced celery
 - 3/4 cup diced carrots
 - 2 cloves garlic, finely chopped
 - 2 teaspoons kosher salt
 - 3/4 cup tomato sauce
 - 2 cups green or brown lentils
 - 2 cups water
 - 4 cups low-sodium chicken or vegetable broth
 - 1 bay leaf
 - 1 sprig fresh thyme
 - 1 teaspoon balsamic or sherry vinegar
 - Vinaigrette:
 - 1/4 cup olive oil
 - 2 tablespoons balsamic or sherry vinegar
 - 1 clove garlic, minced
 - 1/2 teaspoon salt
 
- Heat oil in a large skillet over medium heat. Cook onion, celery, carrots, and garlic with salt, stirring occasionally, until limp and browning, about 10 minutes. Transfer vegetables to a slow cooker with tomato sauce, lentils, water, broth, bay leaf, thyme, and vinegar. Cook on Low for 6 hours.
 - For vinaigrette, whisk together olive oil, vinegar, garlic, and salt.
 - Taste soup for salt, then serve with a drizzle of vinaigrette over each bowl.