Heat the olive oil in a large pot over medium heat. Add the onion and garlic and cook for 3 minutes until the onion softens.
Stir in the carrots and thyme sprigs and cook for 2 minutes to release the herb aroma.
Add the lentils, diced tomatoes with their juice, vegetable broth, salt, and pepper.
Bring the mixture to a boil, then reduce the heat to low and cover the pot.
Simmer for 30 to 35 minutes, stirring once or twice, until the lentils are tender and the carrots are soft.
Remove the thyme sprigs before serving.