Melt butter in a large pot. Add in onion and celery and cook for 7-8 minutes, until tender. Stir in vegetable broth and peas. Cook until peas are just slightly tender, about 3 minutes. Add in mint and parsley.
Using an immersion blender or a food processor, puree the mixture until smooth.
Add heavy cream, salt and pepper to taste. If the soup is too thick, add additional vegetable broth.
Serve warm or let cool for 20 minutes and serve with a dollop of crème fraîche.