Roughly chop the romaine lettuce and toss it into a large bowl.
Dice the tomato and add it to the bowl with the lettuce, then quarter the cucumber and slice thinly, adding it to the lettuce and tomatoes.
Slice the onion in thin rings, then quarter, separating the layers before adding in with the rest of the salad.
Add the olives and feta cheese, along with a generous amount of dressing, then gently toss all the ingredients together and serve immediately.