This soup is very flexible - you can add different vegetables according to your preferences. It can be made ahead and reheated, but you will need to add extra vegetable stock to thin it out because it will thicken as it sits.
Heat a large saucepan over medium-low heat. When the pan is heated, add oil and swirl to coat pan. Add onions and cook until soft, about 3-5 minutes. Don't let the onions brown. If they start to brown turn down the heat. Add garlic and cook for 1-2 more minutes.
Add the tomato paste and stir to coat onions. Cook for a minute, then add all ingredients except the cheese (tomatoes, vegetable stock, fennel, parsley, oregano, basil, ground fennel, additional salt, ground pepper, spinach and lasagna noodles).
Heat soup to simmering point, then simmer for 20 minutes, or until the lasagna noodles are cooked. While the soup is simmering, preheat the oven to 375 degrees F / 190 degrees C. When the noodles are cooked, sprinkle the mozzarella over the soup, then bake in the oven for 5 minutes until the cheese has melted. (You can divide the soup into oven-safe bowls and heat individually, or heat the whole pot.)
Serve hot. Leftovers can be reheated, but you will need to stir frequently as the cheese will be distributed in the soup and will cause sticking. If you want to make the soup ahead of time, you can add the cheese when you are ready to serve.