Preheat the oven to 200°C/ fan180°C/gas 6. Scrub the beetroot, rub with the oil and season with salt. Wrap each 1 in foil and pop into a roasting tray, along with about 300ml water. Roast for 50 minutes or until tender when pierced through with a skewer. Cool in the foil then rub the skins off with rubber-gloved fingers. Slice and put into a large bowl.
Cut the top and bottom off each clementine and, working from top to bottom, remove the skin and pith. Hold the clementines over the beetroot and cut out segments, working between the membranes so the segments fall into the bowl. Squeeze the juice from the leftover membrane into another bowl, discarding any pips. Combine the juice with the extra-virgin oil. Season.
Gently toss the beetroot and clementines together. Layer the salad onto each plate, alternating slices of beetroot, clementine and fish. Drizzle with the dressing and garnish with the mint leaves to serve.