Hot smoked salmon with new potatoes watercress

Name: Hot smoked salmon with new potatoes & watercress
Ingredients: 1 small red onion thinly sliced into half moons; 1 tbsp white wine or rice wine vinegar; 200g baby new potatoes or Jersey royals, halved if large; 2 tbsp buttermilk; 1 ½ tbsp low-fat mayonnaise; 1 tbsp freshly grated horseradish or from a jar; 100g fine green beans trimmed; 1 small sweet red apple cored and sliced; 2 hot smoked salmon fillets (about 300g/11oz), skin removed, broken into pieces; 100g watercress roughly chopped