Hit SAUTE button on Instant Pot, and add the butter and onion. Cook until slightly soft, stirring frequently
Press Cancel, and pour in chicken broth, then diced tomatoes, basil, sea salt, and pepper
Place lid on pressure cooker, turn vent to Sealing, and hit PRESSURE COOk for 5 minutes
Pot will take a few minutes to come to pressure, then begin counting down and cook for 5 minutes
When pot beeps, allow a 5 minute natural release, then do a quick release for remaining pressure
When pin drops, carefully remove lid, and use immersion blender to blend ingredients until smooth.
Stir in the heavy cream, and serve immediately or keep warm until ready to serve. Garnish with fresh basil and shredded Parmesan cheese, if desired