Boil the medium potatoes in water for 15-20 minutes until tender. In a separate pot, hard-boil the eggs for 10-12 minutes. Let both cool completely, then dice them. (You can prepare this step up to 1 day ahead.)
Dice the poultry sausage finely. Grate the radishes and cucumber. Finely chop the parsley and dill. Cut the spring onions into thin rings.
In a large saucepan or bowl, combine the diced sausage, diced potatoes, diced eggs, grated radishes, grated cucumber, chopped parsley, chopped dill, and spring onions.
Add the 2 cups sour cream, 1 cup buttermilk, 1 tablespoon mustard, and lemon juice to taste. Stir gently. (For a vegetarian version, omit the poultry sausage.)
Gradually add the chilled mineral water until the soup reaches your desired thickness (you will likely need 1.5-2 liters total). Season with salt and pepper to taste.
Refrigerate for at least 3 hours before serving (the soup tastes best after overnight). The soup keeps in the refrigerator for up to 3 days.