Add butter to the Instant Pot pan on Saute mode until melted, then add the diced onion and saute until it starts to turn translucent.
Add the garlic, and continue to saute until the onions and garlic are soft.
Add the chicken stock, diced tomatoes, tomato paste, sea salt and pepper, then stir to combine.
Place the lid on the Instant Pot and cook on Soup mode for 5 minutes. When the time is up, allow for natural pressure release for 10 minutes, then release remaining pressure.
Remove the lid and stir in the heavy cream. Use an immersion blender to blend the soup into a smoother texture. For the smoothest texture, you can blend the cream of tomato soup in a blender.
Serve in bowls garnished with fresh basil. Add salt and pepper to taste, if needed.