Add the chopped tomatoes, sliced cucumber, sliced red onion, diced avocado, and chopped cilantro to a large salad bowl, arranging them so the ingredients are loosely layered rather than packed tightly.
Drizzle the salad with the olive oil and fresh lemon juice, making sure to cover as much of the surface as possible. Using a large spoon or salad tongs, gently toss everything together, taking care not to mash the avocado so it stays in soft, distinct chunks.
Just before serving, sprinkle the sea salt and black pepper evenly over the salad and give it one more light toss so the seasoning is distributed throughout. Serve right away for the freshest flavor and best texture.