Step 1

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.

Step 2

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).

Step 3

Reheat the soup if necessary and adjust the consistency with water and seasoning with salt and pepper as needed

Step 4

Serve hot with a dollop of sour cream and toasted chopped walnuts sprinkled on top.