Okroshka, Russian cold soup

To celebrate the end of the summer, a few days ago I have decided to prepare a Russian cold soup called okroshka. Traditionally there are 2 main versions of this cold soup: one is made with kvas (a fermented bread drink, mildly alcoholic) and another one, made with kefir.
It is a refreshing soup, very easy to make and perfect for a lazy lunch on the dacha or in the garden. I haven't tried to make my own kvas, so this is a kefir-based version of okroshka.

Okroshka with kefir
Ingredients:
kefir (tub of 380g)
3 hard-boiled eggs
4 medium potatoes, diced
1 big cucumber, diced (or several mini-cucumbers)
a handful of radishes (about 8), sliced
4tbsp dill, finely chopped
4 spring onions, finely chopped
150g cold ham (like honey roast ham), cubed
salt

To my delight I have discovered that Tesco sells kefir. It is by a Polish brand Bakoma, and comes in a big plastic tub. Tastes identical to the Russian kefir. Last time I bought 3 of them, and they were gone in a couple of days, it is a lovely drink. If you haven't tried it, it tastes like a drinking yogurt. Imagine the Greek style yogurt diluted with water to the runny consistency.

Chopped and diced ingredients for okroshka

Boil potatoes until cooked, for about 20 minutes, don't overcook them, they shouldn't get mushy. Cool them. Dice the cold potatoes. Cook the eggs, they have to be hard boiled, and once cold, dice them as well.
Cube the ham. Some Russians use the boiled beef for this soup (the beef has to cool in broth or it might taste too dry). Some use the sausage like bologna.
Slice and dice all the veg and herbs.
Mix all the ingredients well in a big bowl. Season with salt.
Some cooks prefer to prepare the kefir in advance by stirring in the chopped dill and spring onions and leaving it to infuse for at least an hour or even overnight. I prefer to add the greens to the salad mix, and pour the slightly diluted kefir straight into each bowl.
As kefir is a bit thick in consistency add a bit of cold water to dilute it, just to make it less yogurty.

The ratio of kefir to the mixed veg is a bit like pouring milk over the cereals, i.e. it is not very liquid, but more substantial, maybe 2/3 of veg mix to 1/3 of kefir.

For a vegetarian version, skip the ham. I have also read that some people add the carrots to the soup, but that's not how I make it, and I don't like the sound of carrots added here.