Turn the slow cooker to low to allow it to warm whilst gathering your ingredients.
Saute the onion in olive oil in a small fry pan until lightly browned.
Add the garlic to the pan and cook for 1 minute.
Place the browned onion and garlic into the slow cooker.
Add the stock, carrot, celery, chopped tomatoes, soup mix, and oregano to the slow cooker.
Season well with salt & pepper. Stir thoroughly
Cook for 8 hours on low.
Stir the chopped spinach or silverbeet, and the parsley, through the soup.
Allow to stand for five minutes before serving.