Preheat the oven to 180ºC/350ºF/gas 4.
Place the cod in an ovenproof dish, sprinkle with salt and pepper and bake for 10 minutes. Cool down and carefully remove any bones you find.
Place the cod, potatoes, olive oil and nutmeg in a mixing bowl. Mash the cod mixture with a fork and mix it well together. Season to taste with salt and pepper.
Place the rye bread slices on a work top. Divide the cod and potato mix on each slice of bread, and place radishes and chives on top. Sprinkle with pepper. Serves 4.