Wash and trim the radishes, reserving some for garnish.

Melt butter in a large pot over medium heat. Add onion and cook until softened, about 5 minutes.

Add radishes and vegetable broth to the pot. Bring to a boil, then reduce heat and simmer for 20-25 minutes, or until radishes are tender.

Carefully transfer the soup to a blender (or use an immersion blender) and blend until smooth.

Return the soup to the pot and stir in heavy cream, mint, salt, and pepper.

Heat through and serve, garnished with reserved radish slices.