Melt the butter in a skillet over medium heat. Once melted add the onion and celery and cook until veggies are soft, 8 minutes.
Transfer the veggies to a slow cooker.
Add the basil pesto, canned tomatoes, oregano, and sugar to the slow cooker. Stir to combine. Cover and cook on high for 4 hours.
Transfer the mixture to a blender and blend until smooth. Return the soup to the slow cooker and cook on low or high for another hour. *see notes
Stir in the heavy cream 15 minutes before serving (keep the crockpot on low to warm the cream).
Notes
Slow cooker: I use my smaller sized slow cooker (about 2 qts) for this recipe and it fits perfectly. Blending: Use caution when blending any hot liquids and always follow your equipments instructions included in the manual. I use my Vitamix which has a small circular opening I can remove from the lid. This allows hot air to escape so the hot liquid doesn't spray out when you open the lid. I hold several paper towels in a hot-glove-covered-hand that is gently placed over top of the circular hole of the lid. The paper towel prevents the hot soup from splattering on me while allowing some of the hot air to escape as I'm mixing. *Once you have pureed the soup, you can place it back into the crock pot and immediately stir the heavy cream into the mixture. You don't have to cook it for another hour (it just allows for more time to let the ingredients simmer together). Storing: store leftover soup in an airtight container in the fridge for 5-6 days. Freezing: I don't recommend freezing cream. It's challenging to freeze cream and reheat it without curdling. If you're wanting to freeze this recipe, make it but don't stir the cream into the full batch. Add it to individual servings. Freeze any leftovers (that don't have cream) for up to 3-4 months. Thaw overnight in the fridge and then reheat in a crock pot or in a pot on the stove. Once it's warmed, stir in the heavy cream and serve. Nutritional information is an estimate.