Heat olive oil in a large pot over medium heat. Add onions, carrots, and celery, and sauté for 5-7 minutes until softened.
Add garlic and cook for another 30 seconds until fragrant.
Add rinsed lentils, canned tomatoes, vegetable broth, tomato paste, and all herbs and seasonings (oregano, basil, thyme, bay leaf, salt, pepper, and red pepper flakes if using).
Bring to a boil, then reduce heat to low and simmer uncovered for about 20-25 minutes, stirring occasionally, until lentils are soft and the sauce has thickened.
If the sauce gets too thick, add a little water or broth. If too thin, simmer a bit longer.
Taste and adjust seasonings. Add sugar or honey if the tomatoes are too acidic.
Remove bay leaf and stir in fresh parsley if using.