- In a bowl, whisk together all the ingredients. Season with salt and pepper. Set aside.
- With the rack in the middle position, preheat the oven to 200°C (400°F).
- Grate the potatoes onto a clean tea towel. Wring out as much moisture as possible by twisting the towel over the sink.
- In a bowl, combine the potatoes and onion. Season with salt and pepper.
- Melt the duck fat or butter in a 25-cm (10-inch) ovenproof non-stick skillet over medium heat. Spread the potato mixture in the skillet, pressing down lightly to form a large pancake. Cook for 7 minutes. Do not stir. Invert the rösti onto a large plate and slide it back into the skillet. Transfer to the oven and bake until the potatoes are tender, about 15 minutes.
- Warm the duck legs in the microwave. Debone and shred the meat.
- In a bowl, combine the warm duck, watercress, pine nuts and cherries. Add dressing to taste. Adjust the seasoning.
- Cut the rösti into 4 wedges. Top with the salad.