Heat the butter in a saucepan and cook onion until softened. Pour in the petit pois and add the chicken stock and simmer, uncovered, until peas are tender, about 10 minutes. Add the mint and remove sauce pan away from the heat.
Pour soup into a blender and purée until smooth. Drain through a sieve and discard the solids. Whisk in the cream, salt and pepper. Pour into 4 small stemmed glasses or shot glasses, cover and chill until ready to serve. Any remaining soup is nice heated over a low heat until hot or eaten cold.