Heat a large Dutch oven or another heavy-bottomed pot over medium heat. Add the oil and heat until shimmering. Add the onions, leeks, celery, and carrots to the pot. Sauté for five minutes, or until golden.
Pour in the broth and scrape any brown bits on the bottom of the pot with a wooden spoon. Add the lentils, thyme, bay leaves, pepper, and nutmeg. Cook for 30 minutes on medium, covered.
Add the potatoes and cook for an additional 15 minutes, until the lentils and vegetables are tender. Stir in the vinegar and sugar, then add salt to taste. Serve with warm bread and vegan butter.
Notes
*Substitute mushroom broth or vegetable broth if preferred.Store leftover soup in an airtight container in the refrigerator for up to 5 days. To freeze, store the soup in portions in your freezer for up to 3 months. Thaw in the fridge overnight when ready to use.To reheat, warm it on the stove over medium heat until hot, stirring occasionally. Alternatively, you can reheat it in the microwave. Heat it in a microwave-safe container, stirring occasionally, and continue heating until it's thoroughly warmed.