In a large pot, heat the olive oil over moderately-low heat. Add the onion, cover and cook, stirring occasionally, until softened, about 5 minutes. Add the broth, tarragon, salt and a few turns of freshly ground black pepper and bring to a boil. Add the peas and cook just until defrosted.
In a blender, puree the soup in 2 batches until very smooth. If serving hot, return the soup to the pot and bring just to a simmer. If serving cold, transfer to the refrigerator to chill. Ladle into bowls. Stir the yogurt so that it is smooth and top each serving with a swirl of yogurt.