Preheat the oven to 200 C, 180 Fan, Gas 6. Place the beetroot in a baking tray and toss with the olive oil. Season with salt and pepper and put into the hot oven. Bake for 60-90 minutes until soft and shrivelled.
Meanwhile make the green mayonnaise. Blitz all the ingredients except the mayonnise in a blender until smooth. Transfer to a bowl and mix in the mayonnaise or Alioli. Check seasoning
Serve the beetroot warm or at room temperature with the green mayonnaise on the side.