Heat a large pot over medium heat. Add the ground beef and cook, breaking it apart with a spoon, until browned and cooked through, about 5 minutes. Drain any excess fat if needed.

Add olive oil, onions, celery, carrots, and garlic. Cook, stirring occasionally, until the onions are soft and translucent, about 5 minutes. Do not brown; reduce heat if necessary.

Stir in the diced tomatoes (with their juices), broth, lentils, thyme, bay leaves, salt, and black pepper. Bring to a boil.

Cover partially, reduce the heat to low, and simmer until the lentils are tender, 45 to 50 minutes (see note below if using common brown or green lentils).

Remove the bay leaves and discard them.

Use an immersion blender to purée the soup slightly, leaving some texture for a hearty consistency. If you don’t have an immersion blender, blend about 2 cups of the soup in a blender and return it to the pot.

Garnish with fresh chopped parsley if desired and serve hot.