Put canned tomatoes in a blender and blend up either as smooth or as chunky as you like. Personally, my husband and I enjoy it a bit chunky but for the sake of our kids, I make it pretty smooth.
Meanwhile, if using chopped onion start frying it in a large pot with the butter, until just softened. If using powdered onion, skip this step.
Pour blended tomatoes, chicken broth, sweetener of choice, powdered onion (if no chopped onion) and baking soda into the pot. Simmer for 20-30 minutes.
Add basil and salt and simmer another 5 minutes. Taste and add any additional sweetener or seasonings.
Gently heat cream either in a double boiler (or a bowl set on top of a steaming pot), or simple in a regular saucepan (but then you need to watch it really carefully so you don't scorch it). Heating the cream prevents it from curdling when you add it to the hot soup. Ask me how I know.