Melt butter in a medium pot. Add shallots and cook over medium-high heat for about one minute.
Add stock and mint leaves. Bring to a boil.
Add frozen peas, bring back to a boil and then simmer for about 4 minutes or until the peas are tender. Season to taste. Let the soup cool a little bit. Remove mint leaves, transfer soup to your blender (or use an immersion blender) and blend until very smooth.
If you want to eat this soup cold, set aside in the fridge until ready to eat (it's best eaten very cold). If you prefer it warm, transfer the soup back to the pot and reheat.
