Blanch the vegetables in salted boiling water just until tender, about 3 minutes for the asparagus and 2 to 4 minutes for the beans. Drain and cool in a bowl of ice water. Drain again and refrigerate until ready to use.
Combine the sour cream, lemon juice, parsley, salt and pepper in a bowl and whisk to blend. Keep chilled until ready to use. Serve in a bowl alongside the vegetables.