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.