In a large (5-quart+) slow cooker, pour in the olive oil and tip to coat the bottom of the slow cooker insert.
Add all of the other ingredients – EXCEPT the salt and the pesto – to the slow cooker and stir to combine.
Place the lid on the slow cooker and cook on low for 8 hours.
After 8 hours, remove the lid and stir the soup. Place half of the soup into a blender and process until smooth (leave the other half in the slow cooker). Pour the pureed soup back into the slow cooker with the un-pureed soup and stir to combine.
Stir in the pesto plus 1 teaspoon salt and stir again to combine. Add additional salt and pepper to taste if desired.
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