Heat the olive oil in a large pot over medium heat
Once the oil is glistening, add the onion, celery, carrots, mushrooms, garlic, thyme, salt, and pepper
Cook, stirring occasionally, until the vegetables are soft and beginning to brown, about 10 minutes
Add the stock, 2 cups of water, and the tomatoes
Increase the heat to high and bring the soup to a boil
Add the lentils, then reduce the heat to medium-low and cook until the lentils begin to soften, about 12 minutes.Increase the heat to high, add the quinoa, and bring to a boil
Reduce the heat to medium-low and cook until the lentils and quinoa are tender, about 20 minutes more
Discard the thyme stems.Ladle the soup into bowls
Garnish with a drizzle of olive oil and black pepper
Add a drizzle of balsamic vinegar, Parmesan cheese, and parsley if using
Serve warm with crusty bread alongside if desired
