Preheat the oven to 200C/400F/Gas 6.
Pour the olive oil into a large baking tray and add the garlic and crushed coriander seeds and season with salt and freshly ground black pepper.
Remove the leaves from the baby red beetroots and cook in a pan of boiling salted water for 20 seconds. Drain the leaves and add them to the olive oil mixture. Stir in the lemon zP
Place the red beetroot, golden baby beetroot and candied baby beetroot onto a baking tray and season with salt and freshly ground black pepper. Sprinkle over the paprika and hot paprika and roast in the oven for 10 minutes.
Meanwhile, rub the mackerel fillets with oil and season with salt and freshly ground black pepper.
Add the smoking chips to the smoker and place onto the heat. Place the mackerel fillets into the smoker, close the lid and smoke for 6-8 minutes, according to manufacturer's instructions. (CAUTION: smoking generates a lot of smoke. Cook in a well-ventilated area, and open the smoker outside if possible.)
Remove the beetroots from the oven, cut into quarters and drizzle with the honey.
To serve, remove the beetroot leaves from the oil mixture and scatter onto each of four serving plates. Lay the mackerel fillets on top and arrange the beetroot around the plate. Sprinkle over the fresh horseradish and pea shoots. Squeeze over a little extra lemon juice.