Brown the ground beef in a large pot over medium heat. Break it up with a spoon until no pink remains. Drain excess fat if needed.
Add the diced onion, sliced carrots, and chopped celery to the pot. Cook for 5 minutes until the vegetables start to soften.
Stir in the minced garlic and cook for 1 minute until fragrant.
Add the rinsed lentils, diced tomatoes with their juice, beef broth, dried thyme, and bay leaf. Stir everything together.
Bring the mixture to a boil, then reduce heat to low. Cover and simmer for 30 to 35 minutes until the lentils are tender.
Remove the bay leaf. Taste and season with salt and black pepper as needed. Serve hot.