Bring a large pan of salted water to the boil. Blanch the watercress and the parsley by plunging it into the boiling water and removing it straight away. Drain in a colander to remove any excess water. Melt 20g of the butter in a large saucepan. Add the shallots and garlic and cook over a medium heat for 5 minutes until soft but not coloured. Add the stock and bring to the boil. Add the peas and cook for 2 minutes until the peas are just tender and bright green. Add the cream then remove from the heat. Add the blanched parsley and watercress and blend with a stick blender. Season the soup with salt and pepper and add the remaining 25g butter. Ladle the soup into warm serving bowls and garnish with croutons and sour cream.