I love red onion and tomato together, just as is, with red wine vinegar. Sweet and crunchy with just a little bit of acidity.
Building on this very basic salad, I added a few slices of a very fresh green pepper and a few crumbles of feta. Sprinkle over some oregano, Maldon salt and black pepper, and drizzle with red wine vinegar and olive oil. I also added a spritz of lemon juice.