Heat the butter in a medium saucepan over medium heat. Add the onion and cook, stirring, until tender, 3 to 5 minutes. Add the potato, ginger, thyme, rosemary, 1/2 teaspoon salt and a few grinds of pepper and cook, stirring, until the potatoes are well coated, 2 to 3 minutes. Pour in the broth and bring to a simmer.
Add the beets and cabbage and simmer until the potatoes and vegetables are tender, about 20 minutes, adjusting the heat as needed to maintain a simmer. Remove from the heat, discard the herbs and let cool at least 5 minutes. Carefully puree until smooth with a handheld immersion blender or in 2 to 3 batches in a blender (do not fill the blender more than halfway full).
Reheat the soup if necessary and adjust the consistency with water and seasoning with salt and pepper as needed
Serve hot with a dollop of sour cream and toasted chopped walnuts sprinkled on top.
When blending hot liquid, first let it cool for 5 minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters and pulse until smooth.
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