Waldorf salad with grilled mackerel

Name: Waldorf salad with grilled mackerel
Ingredients: 2 large free-range egg yolks; 1 tbsp white wine vinegar; 2 tsp Dijon mustard; ½ tsp caster sugar; ¼ tsp flaked sea salt; freshly ground black pepper; 150ml/5fl oz sunflower oil; 100ml/3½fl oz crème fraîche; 65g/2¼oz shelled walnut halves; 3 large sticks of celery, plus celery leaves; 2 eating apples, quartered and finely sliced; 250g/9oz seedless red grapes, halved; around 2 tsp fresh lemon juice; 8 fresh mackerel fillets; 4 tsp olive oil; 4 tsp lemon juice; 2 tbsp flatleaf parsley, finely chopped; flaked sea salt and freshly ground black pepper