Lentil-Vegetable Soup

This is a recipe I picked up over the Thanksgiving holiday from my aunt. I made it a few days ago and it's one of those soups that just get better overnight!

Mats
4 tsp olive oil
1 onion, diced
2 carrots, peeled and sliced into 1/4 inch thick rounds
1 stalk celery, chopped into 1/4 inch pieces
2 bay leaves
2 Tb tomato paste
1 tsp salt
2 cloves garlic, finely chopped
3c stock (I used a cup of vegetable stock, a cup of mushroom stock and a cup of chicken stock. I was using up leftovers. Any combination of stocks would work)
3c water
1c dried lentils - try to get French dark green lentils; brown or yellow make the soup look kinda sickly
6c baby spinach
2 tsp red wine vinegar
2 tsp Dijon mustard
1/4 tsp ground black pepper
Parmesan cheese, for topping

In a large pot, heat the oil over medium-high heat. Add the onions, celery, and carrot (this is mirepoix, pronounced meer-pwah, and is a base for many French recipes). Cook on medium-high for 10 minutes, stirring occasionally so the veggies don't stick to the bottom of the pot.
Add the bay leaves, tomato paste, salt, and garlic and cook for 1 minute, stirring constantly.
Add the stock, water and lentils and crank the heat up to high
As soon as the soup starts boiling, reduce the heat to medium and cover. Keep the lid a bit cocked so some of the steam can still escape. Make sure the soup is still boiling, and cook for 25 minutes.
Add the spinach, vinegar, mustard and another cup of water. Stir well and make sure all the spinach is in the liquid, then cover and cook 15 minutes.
Ladle into bowls, remove the bay leaves and top with the cheese, if desired.