In a saucepan cook onion in butter, covered, over moderate heat, stirring occasionally, 5 minutes, or until tender. Add peas and broth and simmer, uncovered, 5 minutes, or until peas are tender. Stir in mint.
In a blender puree soup until smooth and pour through a coarse sieve back into pan. Heat soup over moderate heat until hot and remove pan from heat. Add white pepper and salt, to taste, and stir in all but 4 teaspoons buttermilk.
Divide soup between 4 bowls and gently spoon remaining buttermilk onto each serving.
