Heat oil in large pan over medium-high heat. Add onions, celery, carrots, and garlic; sauté until vegetables begin to brown (about 15 minutes). Add 4 cups broth, lentils, and tomatoes with juice and bring to a boil. Reduce heat to medium-low, cover, and simmer until lentils are tender (about 35 minutes).
Transfer 2 cups of soup to a blender and purée until smooth. Return purée to soup in pan. Season with salt and pepper and a splash of vinegar.
Recipe Notes
Trader Joe’s steamed lentils work great in this recipe and cut the cook time in half!