Turn Instant Pot to Saute setting. Add butter and let melt. Add onions and cook until translucent, about 3-4 minutes.
Mix in tomato paste, allspice, and flour and combine well. Cook 1 minute.
Add in tomatoes, juice, and brown sugar. Mix together.
Switch to manual pressure setting. Cook on high for 25 minutes (ensure top knob is sealed).
When Instant Pot beeps, quick release pressure by turning knob to venting.
Immersion blend until smooth. Pour in cream and serve.
If using frozen tortellini, add to Instant Pot after blending and keep on Warm setting for 1/2 hour to cook through.