In a large saucepan, melt the butter. Add the chopped onion and cook over medium-high heat until translucent.
Add the tomatoes, salt, pepper, baking soda, and oregano to the onions and bring to a boil. Reduce the heat and simmer uncovered for about 15 minutes.
Put the mixture in a blender (or use your hand blender) and purée it. Strain the mixture through a fine-mesh sieve using a spatula to push it through. Be careful on this step because the soup is HOT!
Return the soup to the saucepan over low heat and stir in the orange juice. Add the heavy cream and bring just to a simmer while stirring, you don't want it to boil. At this time, add any salt or pepper if needed.
To create the swirl of cream on top, gently pour the cream on top of the soup before serving. Be careful because it will sink if it is put on too fast. I found that a condiment bottle worked perfectly to make a beautiful swirl.