Cold kefir vegetable soup (Okroshka)

1. Boil the potatoes in their skins and hard-boil the eggs for 10 minutes; cool both completely and peel.

2. Cut the potatoes, eggs, cucumbers and ham into small, even cubes (about 0.5 cm) and the radishes into thin slices or strips.

3. Chop the spring onions and dill finely and crush them lightly in a bowl with a pinch of salt using a spoon, so they release their aroma.

4. Whisk the kefir smooth with the cold water, sour cream and mustard and season with salt; it should be thinner than yoghurt but full-flavoured.

5. Put everything into a large bowl, pour over the kefir mixture, stir and chill for at least 1 hour so it's ice-cold.

6. Ladle into deep bowls and serve sprinkled with fresh dill, with rye bread.

Tip: Okroshka is the cooling soup of the Russian summer; in Russia it's often made with kvass, but we've chosen the kefir version – it's alcohol-free, and its tangy flavour is refreshing on hot days.