Peel and dice the beets, apple and the white parts of the spring onions into ½” pieces. Heat a small stockpot over medium heat. Add one tablespoon olive oil to the pot then the beets, apples and spring onions and sweat for about 10-12 minutes until the vegetables start to get tender. Add 1 quart of water, bring to a boil then reduce heat so that the soup is simmering gently. Cook for about 20 minutes, until the vegetables are entirely tender. Season with salt and pepper. Add the soup to a blender and puree on high, for about 2 minutes, until smooth. Pour back into the pot or a bowl and chill in the fridge for at least an hour. Using a sharp knife, cut the peel off of the grapefruit, it will also take the outer layer of white pith. Slice the grapefruit into small segments. Serve the soup topped with grapefruit segments, a drizzle of yogurt, the chopped green parts of the spring onions and parsley.