This is a simple mixture of beetroot (very sweet) and raw onion, with salad dressing on top. It sounds far too strong a taste mix! But the tastes seem to tone each other down.
Quantities of each are up to you. You tend to have a pack of beetroot, and an onion...
Beetroot is sold ready cooked, often in a plastic vacuum pack. This is quite OK to use.
If you need to cook it yourself (if you grow it, for example) then pull off the leaves, and wash the earth off. Put the beets, uncut, untrimmed, unpeeled, into a saucepan with water to cover, and boil. How long depends of the size of the beets - it can take up to 2 hours. You don't peel or cut them as then the colour bleeds into the water. Once cooked, and cooled, the skins will just rub off the beets with no trouble (a rather satisfying experience).
