Clean, rinse and drain the spinach. Rinse the yellow pepper, remove the seeds and white ribs and cut into thin strips. Core the tomatoes and blanch in a pot of boiling water for 10 seconds. Peel and quarter the tomatoes and cut into bite-size pieces.
Peel the potatoes and cut into 1-2 cm (approximately 1/2-3/4-inch) cubes. Peel and chop the onion and garlic. Remove the core from the cabbage, rinse, drain and cut into thin strips. Rinse and thinly slice the celery. Peel the celery root, the carrot and the beet and cut them all into strips.
In a saucepan, combine the potatoes, carrot, pepper, onion, cabbage, celery, celery root and garlic. Pour 750 ml (approximately 3 cups) of water, over the vegetables and season with salt and pepper. Simmer until the potatoes are soft, about 20 minutes.
Add the chopped tomatoes and the spinach to the soup. Stir in the whey and the tomato juice and season the soup with salt and lemon juice. Serve garnished with parsley to taste.