Rinse lentils under cold water and pick through to remove any debris. Set aside.
Heat olive oil in large pot over medium heat. Add bacon and cook until crispy, 7-8 minutes. Remove bacon with slotted spoon and set aside, leaving drippings in pot.
Add onion, carrots, celery, and garlic to bacon drippings. Sauté for 5 minutes until vegetables soften.
Stir in lentils, broth, bay leaf, and thyme. Bring to boil over high heat.
Reduce heat to low and simmer partially covered for 35-40 minutes, stirring occasionally, until lentils are tender.
Remove bay leaf. Season with salt and pepper to taste, starting with less than you think you need.
Stir most of cooked bacon back into soup, reserving some for garnish. Serve hot topped with reserved bacon and fresh parsley.
Notes
French lentils (lentilles du Puy) hold their shape better than regular lentils.
For vegetarian version, omit bacon and use vegetable broth. Add smoked paprika for depth.
Soup thickens as it sits. Add more broth when reheating to thin as needed.
Store bacon separately if meal prepping to keep it crispy.