Grate the apple, carrot, beetroot and celeriac using a coarse grater or the grater attachment on a food processor and put into a large mixing bowl.
Finely slice the red cabbage as thinly as you can and add to the other fruit and vegetables in the mixing bowl. Add the toasted mixed seeds and sultanas and toss everything together well.
Mix the red wine vinegar, olive oil and Dijon mustard together in a small bowl and pour over the slaw and toss well until everything is coated.
Divide the slaw between plates, then add the trout fillets. Serve with lemon wedges to squeeze over the fish and some crusty wholemeal bread, if you like.
Cook's tip: the slaw will keep, chilled, for up to 3 days.