Bring a large saucepan of salted water to the boil and parboil the potato for 10-12 minutes until just tender. Pre-heat the oven to 190 degrees.
Drain the potatoes and allow them to cool to one side for a few minutes.
Combine the remaining ingredients apart from the thyme in a ceramic, rectangular roasting/gratin dish.
Add the potato and toss well.
Bake for 15-20 minutes until the fish is cooked through. Remove from the oven and allow to sit for a few minutes before garnishing with thyme leaves and sprigs of thyme and serving.