Heat oil over medium heat in a large soup pot. Once warm, add the onion and a ¼ teaspoon salt. Cook until translucent, about 5 minutes.
Add the garlic, celery, and carrots, along with an additional ¼ teaspoon of salt. Cook until just starting to soften, about 7 minutes.
When the vegetables have softened a bit, add the mushrooms, tomato paste, and wine. Cook (stirring occasionally) until the mushrooms start to soften and the wine soaks up. About 7 minutes.
Add the crushed tomatoes, vegetable broth, French lentils, rosemary sprigs, bay leaves, ½ teaspoon salt and ½ teaspoon black pepper. Bring the soup to a boil, then reduce to low (just above a simmer) and partially cover the pot with lid.
Let the soup simmer until the lentils are tender, about 50 minutes. While the soup is cooking, use this time to warm up some bread, open a bottle of wine, clean up the dishes, or light some candles.
When the lentils are tender, discard the rosemary and the bay leaves. Use a ladle to transfer roughly 2 cups of the soup to a blender to purée. Alternatively, you can use an immersion blender to partially purée some of the soup. Be careful not to blend too much here. You want to maintain the integrity of the lentils in the soup.
Stir in the balsamic. Taste and adjust for salt and pepper if needed. When serving, garnish with chopped fresh parsley and microgreens if desired.