Preheat oven to 180°C. Shred potatoes and apple using a mandolin or grater. To prevent browning, place into a bowl of iced water and juice. Drain well and pat dry, pressing firmly with a clean tea towel. Return to the bowl. Add sage, ginger and egg and season to taste. Toss to coat evenly.
Heat a large non-stick frying pan over medium-high heat. Add 1/4 of the butter. When butter has melted, add 1/2 cup portions of the potato mixture and press to flatten. Cook for 5 minutes until base is golden. Flip over and cook for about 4 minutes until golden. Transfer to oven to keep warm. Repeat with remaining butter and mixture.
Serve with smoked salmon, crème fraîche, extra apple slices and garnish with fried sage leaves (see tip).
In a small frying pan, fry sage leaves, a few at a time, in hot olive oil until crisp. Drain on paper towel.