Heat olive oil in a heavy-bottomed pot over medium-low heat. Add onion and cook until translucent (about 5 minutes), then add celery and carrots. Cook this mixture for 10-15 minutes until vegetables have softened and reduced in size, stirring occasionally.
Stir in minced garlic and cook just until fragrant, about 1 minute. Add oregano and bay leaves.
Add lentils, tomatoes (with their juice), and vegetable broth. Bring to a gentle boil, then reduce heat to maintain a lazy simmer. Cover partially and cook for 45-60 minutes, stirring only once every 15 minutes.
Remove bay leaves, season with salt and freshly ground pepper to taste. If desired, stir in a handful of chopped fresh parsley.
