Heat olive oil in a large pot over medium heat.
Add chopped onion, carrots, and celery.
Cook for about 5 minutes, until vegetables are softened.
Add minced garlic, dried thyme, and black pepper.
Stir and cook for 1 minute, until fragrant.
Add rinsed lentils, water or broth, and bay leaf if using.
Stir to combine.
Bring the soup to a boil over high heat.
Once boiling, reduce heat to low and cover the pot.
Simmer for 25 to 30 minutes, or until lentils are tender.
Remove the bay leaf.
Stir in salt and adjust to taste.
For a thicker soup, mash a portion with a spoon or use an immersion blender briefly.
Serve hot with optional toppings like parsley, cheese, or crusty bread.