Wash & dry your fresh basil. Add to a blender or food processor with pine nuts, Parmesan cheese, garlic & olive oil. Blend until a smooth paste is formed.
In a large stock pot, melt your butter. Add celery & onions, cook just until translucent. Add in dried oregano. Stir in pesto and tomatoes.
Transfer to your slow cooker and cook on low 8-10 hours or high 4-6.
With an immersion blender, blend soup until creamy. Just before serving add in your half and half, mix well to combine.
