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.