Thaw the spinach. Cut off the cress, cut the onion into small cubes and the potatoes into larger cubes.
Melt the butter in a saucepan, add the onion cubes and sweat. Sweat the potatoes as well. Add the stock and cream and let simmer for about 10 minutes. Then add the spinach and cress to the pot, simmer for another 5 minutes.
Let the soup cool slightly and stir in the horseradish. Finely puree the soup with a hand blender and season with salt, pepper and sugar.
Serve the spinach soup in glasses and cups, with the sprouts on top. It is served with breadsticks wrapped in bacon.