Place lentils in a bowl, cover with cold water, and remove any broken lentils that float to the top.

Drain the lentils.

Heat olive oil in a large pot over medium heat.

Add chopped onions and cook until soft and translucent (5-7 minutes).

Add minced garlic and cook until fragrant (about 30 seconds).

Stir in tomato paste, chopped parsley, chopped celery, salt, and bay leaves.

Cook and stir for 3 minutes.

Pour the drained lentils and 8 cups of water into the pot.

Bring to a boil, then reduce heat and simmer partially covered until lentils are soft (30-35 minutes).

Stir in Dijon mustard and red wine vinegar.

Puree the soup with an immersion blender until mostly smooth but still contains some chunks.

Season with salt and pepper to taste.