Soak the onions in the red wine vinegar for about 5 minutes.
Mix the chunks of cucumber, tomatoes and avocado in a bowl.
Add salt, black pepper, olive oil and lemon juice and mix.
Take onions out of the vinegar and mix with the other vegetables.
Taste and adjust if necessary.
Serve cold or at room temperature.
Video
Notes
Depending on the type of cucumber you are able to get, you may be able to leave the skin on and the seeds in. If you are unsure which type you have, a simple taste test before you cut up the rest of the cucumber will solve that problem for you.
This salad does not need a salad dressing. However, if you absolutely want one then this yoghurt dressing is a great choice.
If you don't like raw onions, you can easily substitute them with some onion powder.