Sweet golden beets are the basis for this soup, which is lighter than traditional borscht. If you have the time and inclination, a swirl of red beet purée makes an attractive garnish. The soup stores well and will keep in the refrigerator, covered, for up to 3 days.
Heat the oil or butter in a large saucepan or stockpot over medium heat, and add the beets and onion. Stir to coat the vegetables, then cover the pan and cook for 10 minutes. Add 2 cups of the chicken stock and 1 cup of water, and bring the mixture to a boil. Reduce the heat to maintain a simmer, cover the pan, and cook until the beets are very tender, 45 to 55 minutes. Let cool slightly.
Purée the soup in a blender or food processor until very smooth. If the soup is too thick, thin with some of the remaining chicken stock. Return the soup to a clean pan and reheat over low heat. Add the lemon juice, and then season to taste with salt and pepper.
To make the red beet purée (for the optional garnish), peel a large beet and cut it into 1/2-inch dice. Place in a small pan and cover with water, then cook, covered, until the beet is very tender. Drain and purée until smooth, thinning slightly with chicken stock. Pour the beet purée into a plastic squeeze bottle.
To serve, divide the hot soup among bowls. Garnish each serving with a swirl or dollop of red beet purée, if desired.