Melt the butter in a large saucepan, add the potatoes and onions, cook for 5 minutes on a low level, stirring constantly. Add lemon juice, chicken stock, diced apple and allspice. Bring the mixture to the boil and simmer over a low heat for 15 minutes. Add rocket and cook for another 10 minutes.
Puree half of the soup and stir back into the rest of the soup. Season to taste with salt and pepper. Garnish with apple slices, rocket, and spring onions.