There are two schools of okroshka—those made with kefir, and those made with kvas (a bread-based, mildly fermented drink). Every time I see the kvas version, I order it. Maybe this time I'll fall in love? But so far, it's been a bust. Frankly, it's two tastes that don't seem to belong together—like a salad that somebody poured their soda into.
But kefir-based okroshka—this makes sense. Tangy, creamy, crunchy, with a hint of smoke from the meat. I love it as an alternative to the usual turkey sandwich sack lunch, or as a light summer dinner. I tend to make this soup in the heat of summer, when the garden is bursting with edible flowers. If you have some at your disposal, toss them in the bowl as well.
Place the potato in a small pot, and add water to cover by 1 to 2 inches. Bring to a boil over high heat, then reduce the heat until it's just high enough to maintain a simmer. Cook until the potato is tender when pierced with a knife. Drain and let the potato cool to room temperature.
While the potato is cooking, take 5 of the hard-boiled eggs (set aside the remaining egg for garnish), and separate the yolks from the whites. Dice the whites into ⅓-inch pieces, and set aside.
Place the yolks in a large bowl, and mash them with a fork. Add the mustard, and keep mashing to form a uniform, lump-free paste. Whisk in the smetana, then the kefir, then the water. Season to taste with salt, and refrigerate.
In a large bowl, mix together the reserved egg whites, potato, salo, radishes, cucumbers, scallions, and dill. Mix well, and season with salt and pepper. Place ¾ cup of the salad mixture into soup bowls. Add 1 cup of the kefir mixture into each bowl, then top with slices of the reserved hard-boiled egg, scallions, and dill. Serve very cold. If you're not eating all the okroshka at once, store the kefir base and the prepared vegetables separately, and combine them just before serving. The kefir mixture will keep for up to 1 week, but the vegetables are best mixed up the day they are to be used.