Heat the oil in a large saucepan over a low heat. Add the garlic and sauté until lightly golden. Add the leeks and celery and continue to cook, stirring, for 5-10 minutes until soft but without colour.
Mix in the peas, vegetable stock and mint, then bring to a simmer. Cook gently for 20 minutes. Using a hand blender, purée the soup in batches until smooth. Divide between bowls and swirl through a little crème fraîche.