Wash 300g of raw mixed-colour beets, cook in boiling salted water for 30 minutes, or until just tender (depending on their size), then drain and leave until cool enough to handle.
Meanwhile, cook 150g of bulgur wheat according to the packet instructions, and drain.
Lay a sheet of wet greaseproof paper in a 25cm x 30cm baking tray and rub it lightly with oil. Evenly spread the bulgur over the tray and place 4 fish fillets randomly on top. Loosen 2 teaspoons of Dijon mustard with a splash of vinegar, then brush over the skin side of the fish, and sprinkle over 1 tablespoon of porridge oats.
Remove the skin from the beets and finely slice them, ideally on a mandolin (use the guard!), then arrange them around the fish, overlapping the slices and tucking them up to and under the fillets in a nice even layer.
Mix 1 tablespoon of vinegar with 1 teaspoon of oil and a pinch of sea salt and black pepper to make a dressing.
Holding ½ a bunch of fresh thyme like a brush, use it to dab the dressing all over the beets, then strip over the leaves. Cook at the top of the oven for 15 to 20 minutes, or until the fish is lightly golden and cooked through.
Toss 2 large handfuls of rocket with a squeeze of juice from ½ a lemon and sprinkle over the top. Stir 2 teaspoons of jarred grated horseradish through the 2 tablespoons of natural yoghurt, season to perfection, then spoon over the fish and tuck in.