Heat the oven to 200°C/Fan 180°C/Gas 6. Tip the coriander seeds into a dry frying pan. Toast for 2-3 mins till popping and fragrant. Tip into a mortar and lightly crush with the pestle. Transfer to a bowl.
Finely grate the beetroot. Peel and grate the horseradish. Finely chop the dill stalks (save the leaves for later). Pop into the bowl with the coriander seeds. Pour in 2 tbsp brown rice vinegar. Season.
Roughly chop the parsnips. Cut the leeks in half lengthways and wash out any grit. Finely slice. Pop both into a pan and cover with boiling water. Simmer for 10 mins till soft.
Lay the rainbow trout fillets on a baking tray lined with foil. Cover the trout with the beetroot mix. Slide into the oven and cook for 10 mins till the fish is cooked through. The flesh will flake when pushed with a fork.
Zest and juice the orange. Roughly chop the dill leaves and set aside for later.
Drain the parsnips and leeks. Tip back into the pan and mash till the parsnips are smooth. Season.
Toss the watercress with the orange juice. Serve with the beetroot rainbow trout and the parsnip mash. Scatter over the dill leaves and orange zest.