Heat the butter in a large saucepan, add the fennel and onion and cook over medium heat for 10 to 15 minutes, until the vegetables are tender. Add the chicken stock, increase the heat to high, and bring to a boil. Add the peas and cook for 3 minutes, until the peas are tender. Off the heat, add the salt and pepper.
Puree the soup in batches with a food processor fitted with a steel blade or with a stick blender. Pour the soup into a large bowl and repeat until all the soup is coarsely pureed. Whisk in the creme fraiche and taste for seasoning. Reheat the soup in the saucepan and serve hot with a drizzle of olive oil and some croutons on top.
Heat the olive oil in a large (10-inch) saute pan until hot. Add the bread and cook over medium heat, tossing frequently, until the bread is evenly toasted and browned. Sprinkle with salt and pepper.