Slice tomatoes and cucumbers and place them in a large bowl. Thinly slice onion and add them to the salad bowl.
In a small ramekin, stir together 2 Tbsp sunflower oil, 1 Tbsp vinegar, 1 tsp fine sea salt and 1/4 tsp pepper.
Just before serving, drizzle your salad with dressing and add more salt to taste if desired. Don't add the dressing early or the tomatoes will be too juicy.