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Rinse the tomatoes, remove the hard part and roughly chop the tomatoes. You can add them directly to the slow cooker while cutting the remaining tomatoes.
Peel and roughly chop the onions and garlic cloves. Add this to the slow cooker along with 2 cubes of vegetable stock.
There's no need to add water, there is enough moisture in tomatoes. Set the slow cooker to the lowest setting and let it cook slowly for about 8 hours.
As soon as the tomatoes, onion and garlic have softened, you can puree them. The easiest way is to use an immersion blender for this. A blender is also possible, but that is less convenient.
Once the soup is smooth, you can optionally add macaroni or vermicelli and set the slow cooker on the highest setting for another half hour.
Season the soup with salt and pepper and serve with (fresh) oregano if desired. Enjoy!