In a large Dutch oven, heat up butter over medium heat. Add minced garlic and chopped onion and saute, stirring often, until the onion is just becoming translucent.
Add Italian seasoning and saute for 2 minutes, stirring often.
Add vegetable stock, crushed tomatoes and diced tomatoes to Dutch oven. Stir well.
Bring to simmer and cook for 5 minutes.
Add tortellini and cook until done, about 7 minutes. Check after 5 minutes. Stir once in a while.
Season with salt and pepper.
Remove soup from heat. Stir half and half or heavy cream if using.
Serve garnished with chopped parsley and/or grated Parmesan or mozzarella cheese.
Notes
You can use one tablespoon of butter and one tablespoon of olive oil in step 1.