Wash and core the tomatoes. Place them in a large stockpot over medium-low heat or overnight in a crackpot on low. Cook until the peels have come off the tomatoes and water and pulp is released, 1-2 hours on the stove or overnight if using the crockpot. Remove from heat and allow to cool until you can safely handle the tomatoes.
Secure a mesh strainer over a large bowl. Pour the tomatoes through the strainer in batches. Use a spatial or wooden spoon to work the juice through the strainer. Scrape the pulp off the bottom of the strainer frequently to keep things flowing. Keep mashing the mixture trough the strainer until all that is left are the seeds and peels. When all the tomatoes have beans strained discard the pulp.
Pour the tomato sauce back into the pot and return to the heat, cooking until the sauce is reduced to the thickness you want. You can refrigerate it overnight and make the soup the next day. Using a turkey baster, you can draw the watery part off the top of the sauce before making the soup. This only works if it has been sitting overnight. If making the soup the same day, just cook a bit longer to remove more water.
Add the oil to a medium-sized pot over medium-high heat. Add the diced onion and cook until translucent. Add the garlic and cook for an additional minute but be careful not to brown the garlic. Add four cups of the fresh tomato sauce to the pot. Season with salt, pepper, basil and crushed red pepper to taste. Cook over low heat for 15-20 minutes. Taste the soup halfway through, If it is too tart add a pinch of granulated sugar. Remove the soup from the heat, garnish with sour cream, yogurt, or creme fraiche and extra basil. Serve with a grilled cheese sandwich or crostini with melted cheese. Serve with additional Parmesan on the side.