Cut avocados in half, working around the pit. Separate the 2 halves. Remove pit and scoop out avocado flesh with a spoon. Wedge avocados and pile in the center of a large platter. Seed and wedge tomatoes and arrange around the avocados. Spread sliced onion over platter. Sprinkle platter with cilantro. Squeeze the juice of 2 limes evenly over vegetables. Season with coarse salt. Drizzle platter with oil and serve.