Borscht, or some variation of it, has been a mainstay of peasant food all over Eastern Europe and Russia. In the winter, the soup is served hot and may contain beets, carrots, and potatoes or other root vegetables; in the summer, the same soup (usually called chlodnik*) may be served icy cold and mixed with cream, yogurt, or buttermilk and garnished with sliced radishes, beet greens, cucumbers, lemon slices, and hard-boiled eggs.
Serve this soup hot or cold. Purée it or leave the vegetables whole.
In a large sauté pan, melt the butter over medium heat and sauté the onion for about 10 minutes, or until lightly browned.
Place the onion, beets, potato, carrot, and water in a 7-quart slow cooker. Cover and cook on low for about 6 hours, or until the beets are tender when pierced with a fork.
Using a handheld immersion blender, blend the vegetables until smooth. Add salt to taste. Ladle the soup into bowls. Serve hot, garnished with sour cream and fresh dill.