Prepare Lentils: Soak lentils for about 1 hour, then drain using a strainer.
Prepare Vegetables: Blanch the tomato briefly in boiling water and peel it. Peel and chop celery, chop carrot, onion, and coarsely chop garlic.
Sauté Aromatics and Vegetables: Heat olive oil in a pot, stir-fry chopped garlic until fragrant. Add onion, carrot, and celery and stir-fry over medium heat for about 5 minutes until onion becomes translucent.
Add Tomato, Lentils, and Sauce: Add peeled tomato, soaked lentils, and tomato sauce to the pot and stir-fry together.
Simmer Soup: Add water and bay leaf, bring to a boil, then simmer for about 30 minutes until lentils are soft.
Season and Garnish: Remove bay leaf, season with salt and black pepper, then sprinkle chopped parsley on top.
Serve: Serve hot, optionally with toasted baguette bread.