In a large, heavy-bottomed pot, heat the olive oil over medium heat. Add the onion, garlic, crushed red pepper and salt, and cook until the onions are softened and translucent, 4-5 minutes. Stir in the tomato paste and cook until it darkens and starts sticking to the bottom of the pot, 1-2 minutes longer. Add the vegetable stock, diced tomatoes, and basil.
Bring the soup to a boil over high heat, then reduce the heat to medium low and simmer for 10 minutes. Add the tortellini and simmer until tender, about 5 minutes more. Turn off the heat and stir in the spinach.
Sprinkle with parmesan cheese if desired and serve.
Tip: Freeze the tomato soup base for a quick, easy weeknight meal. All you'll have to add in when ready to eat is the cheese tortellini and spinach!