Heat olive oil in a large pot or Dutch oven over medium heat.
Add the chopped onion, carrots, and celery. Cook until softened, about 5-7 minutes.
Stir in the minced garlic and cook for another minute until fragrant.
Add the rinsed lentils, vegetable broth, thyme, rosemary, and bay leaf.
Bring the soup to a boil, then reduce heat, cover, and simmer for 30-40 minutes, or until lentils are tender.
Remove the bay leaf. Season with salt and pepper to taste.
Serve hot.
Notes
For a thicker soup, mash some of the lentils against the side of the pot with a spoon. You can also add a splash of lemon juice at the end for brightness.