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
Kosher salt
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
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)