Okroshka (Cold Vegetable and Meat Soup with Kvas)

Place the beef brisket in a large pot and cover with cold water. Bring to a boil, reduce heat, and simmer for about 1 hour or until tender. In a separate pot, boil the potatoes in salted water until fork-tender, about 20 minutes. Boil the eggs for 10 minutes, then cool under cold water.

Remove the beef from the pot and let cool. Peel the potatoes and eggs once cool. Refrigerate all cooked ingredients until cold, about 30 minutes. Chilling is essential for authentic cold soup texture.

Cut the cooled beef, potatoes, cucumbers, radishes, and eggs into small uniform cubes (about 1/2 inch). Slice the green onions thinly. Finely chop the dill.

In a large bowl, mix together the diced beef, potatoes, cucumbers, radishes, eggs, green onions, and half of the dill. Season with salt and pepper. If using mustard, stir it in with a spoonful of sour cream to create a paste before adding.

Pour the cold kvas over the vegetable-meat mixture. Add the remaining sour cream and stir gently to combine. Adjust salt and pepper to taste. The soup should be tangy and refreshing.

Refrigerate for at least 30 minutes before serving to allow flavors to meld. Serve cold, garnished with the remaining dill and optional halved boiled egg. Add extra sour cream on top if desired.