First peel the apple and pumpkin, cut in half, core and dice. Melt the butter in a large saucepan and add the onions. Fry until glassy and then add the 1 teaspoon of curry powder. Fry for about 2 minutes, stirring constantly, until the mixture smells intensely.
Add the chicken or vegetable stock, pumpkin, apple and sage and bring to the boil. Turn the heat down and simmer for about 20 minutes until the pumpkin and apple are soft.
Meanwhile, whip the cream double in a bowl until stiff, fold in the horseradish and the rest of the curry powder, cover and refrigerate.
Puree the soup with a blender, add apple juice and season with salt and pepper. Scoop the soup into plates and add a dollop of horseradish cream. Use lime zest for garnish.