Peel and cut the onions, carrots, and beets (alternatively, shred the carrots and beets using the shredding blade of a food processor) and sauté over medium heat in the olive oil with a pinch of salt in a large soup pot. Reserve a small amount of beet to grate and add near the end to enliven the color. In the meantime, bring the vegetable stock to a boil. When the vegetables are soft (about 5 minutes), add the shredded cabbage and the hot stock. Bring to a boil and simmer 15-25 minutes, until the vegetables are tender. With a few minutes left, add the reserved grated beet. Season to taste with salt and pepper, then squeeze in the lemon juice, aiming for a pleasing but subtle sour taste. Serve with freshly grated black pepper, a dollop of sour cream, and chopped parsley, if desired.