On medium heat, add butter to a Dutch oven or soup pot.
Once the butter has melted and is foaming, add onions and sauté for a few minutes until the onions soften.
Add garlic, salt, and red pepper flakes (if using) to onions and stir well.
Add sweetener to the onion mixture and stir well.
Add tomatoes to the onion mixture and stir well. You can also begin to crush the tomatoes if you want.
Add liquid to the tomato and onion mixture and stir well. Heat to a simmer on medium-high heat.
Once the simmer is reached, cover the soup pot and turn the heat to low. Allow the soup to simmer covered for 45 minutes to one hour.
After simmer time, puree to the desired texture using a handheld stick blender or a traditional blender. (If using a traditional blender, puree in batches and be sure to allow the steam to escape.) (See the recipe video to see how to use the stick blender.)
After pureeing, the soup is ready to be served.
Refrigerated in an airtight container, tomato soup will stay fresh for 3-4 days.
Frozen in an airtight container, tomato soup will stay fresh for 6 months.