Czech Vegetarian Sauerkraut Soup - Very Good Cook

Ingredients

• 1½ cups (145 g) strained, roughly chopped sauerkraut
• 7 cups cold water
• 2 cups (280 g) diced potatoes, skin-on or peeled, about 1/2-inch dice (yellow, russet, or red potatoes)
• 1 cup (227 g) sour cream
• 2 tablespoons (22 g) unbleached all-purpose flour
• 1 teaspoon salt
• 1 egg, lightly beaten
• 1 tablespoon (15 g) sugar (optional)
• Freshly ground black pepper, to taste (optional)
• Soft-boiled, 6-minute egg (optional)