Heat oil or butter in a large pot over medium heat. Add onion, carrot and celery with a pinch of salt. Cook until soft and the onion is translucent, about 6–8 minutes.
Stir in garlic, thyme and bay leaf; cook 30 seconds.
Add lentils, tomatoes (if using) and broth. Bring to a boil, then reduce to a simmer and cook, partially covered, until lentils are tender, about 25–30 minutes.
Remove bay leaf. Season with salt, pepper and stir in lemon juice. Garnish and serve with crusty bread.
Notes
No soaking required for French green lentils.
Pulse a cup of soup in a blender for a creamier texture, then stir back in.
Freezes well for up to 3 months. Thaw overnight in the fridge before reheating.