Heat the olive oil in a large pot or Dutch oven over medium heat. Add the chopped onion, carrots, and celery, and cook for about 5-7 minutes, until softened.
Add the minced garlic and cook for another minute, until fragrant.
Add the rinsed lentils, diced potatoes, vegetable broth, diced tomatoes (with their juice), bay leaf, and dried thyme to the pot. Stir everything well to combine.
Increase the heat to high and bring the soup to a boil.
Once boiling, reduce the heat to low, cover the pot, and simmer for 20-25 minutes, or until the lentils and potatoes are tender.
Remove the bay leaf. Season the soup with salt and black pepper to taste.
Ladle the soup into bowls and serve hot.