For pickled fennel, combine vinegar and sugar in a small saucepan and stir over low heat until sugar dissolves. Stir through spices. cool to room temperature (20-30 minutes). Add fennel and onion, stir to combine, transfer to a non-reactive container, cover, refrigerate for at least 2 hours.
Place a large steamer basket over a large saucepan of boiling water. Place trout in a single layer on a plate, season to taste, then place in steamer basket. Cover and steam until just cooked through (8-10 minutes). Serve hot with pickled fennel and mayonnaise.