Start by cutting the onion into thin slices.
Then, slice the tomatoes into even slices.
Combine the sliced tomatoes and onions in a bowl.
Squeeze the juice from half a lemon over.
Drizzle with olive oil and season with sea salt.
Mix everything together gently to avoid breaking the tomatoes.
Place the bowl in the refrigerator and let the salad chill for a couple of hours.
Just before serving, optional, sprinkle with chopped parsley.