In a large saucepan, melt 2 Tbsp butter over medium heat. Add in the celery and onion, and cook for around 5 minutes.
Pour the celery mixture and 1 cup of vegetable broth into a blender and pulse until smooth.
Meanwhile, melt the remaining butter in the same saucepan. Mix in the flour and whisk until a paste forms.
Add in the blended celery mixture to the flour paste and whisk until smooth. Slowly add in the remaining vegetable broth.
Cook the soup until it starts to boil. Reduce the heat and mix in the heavy cream. Season with salt and pepper to taste.