Steamed coral trout with pickled fennel

Name: Steamed coral trout with pickled fennel
Prep time: 10 min
Cook time: 10 min
Total time: 20 min
Cuisine: Australian
Ingredients: 4 coral trout fillets, skin on, (about 220gm each); To serve: good-quality mayonnaise; Pickled fennel; 250 ml (1 cup) cider vinegar; 55 gm (¼ cup) caster sugar; 2 tsp black mustard seeds; 1 tsp celery seeds; 300 gm baby fennel (about 1), shaved on a mandolin; ½ Spanish onion, shaved on a mandolin