1 tablespoon olive oil
1 medium celery stalk, small dice
1 medium carrot, peeled and small dice
1/2 medium yellow onion, small dice
3 medium garlic cloves, minced
Freshly ground black pepper
1 quart low-sodium vegetable broth
1 (15-ounce) can diced tomatoes with their juices
1 1/4 cups lentils (any color except red), rinsed
3 medium potatoes cut in chunks
1 bay leaf
1/4 teaspoon finely chopped fresh thyme leaves
1 teaspoon red wine vinegar or sherry vinegar
2 ounces spinach leaves (about 1/2 a bunch)
Heat the oil in a large saucepan over medium heat until shimmering, about 3 minutes. Add the celery, carrot, and onion and cook, stirring occasionally, until the vegetables have softened, about 10 minutes. Stir in the garlic and cook until fragrant, about 1 minute. Season with several generous pinches of salt and pepper.
Add the broth, tomatoes with their juices, lentils, bay leaf, and thyme and stir to combine. Cover and bring to a simmer, about 15 minutes. Once simmering, reduce the heat to low and continue simmering, covered, until the lentils and vegetables are soft, about one hour more.
Taste and season with more salt or pepper as needed, then stir in the vinegar. Add the spinach and stir until wilted. If you prefer a creamier texture, purée half of the soup in a blender and add it back to the pot.