Step 1Cut the onion in quarters lengthwise, then cut each quarter into lengthwise halves or thirds. Cut each of these sections in half crosswise. Place the onion pieces in a bowl of cold water to soak for at least 10 minutes.
Step 2Cut the avocados in half, remove the pits and, with a spoon, scoop out the flesh in one piece. Cut the avocados into pieces roughly the same size as the onions.
Step 3In a large bowl, whisk together 2 tablespoons of lemon juice, the olive oil, salt and chives to mix well. Drain the onions and pat them dry with a towel. Add the onions to the dressing and stir to coat well. Remove the onions with a slotted spoon, draining any excess dressing back into the bowl, and arrange the onions on a platter in a broad layer.
Step 4Place the avocados in the remaining dressing and stir to coat well. Remove them with a slotted spoon and arrange them in an oblong mound on top of the onions, centering so there is onion showing around the edges.
Step 5Add the shrimp to the remaining dressing, adding another squirt of lemon juice and a little more salt to taste. Arrange the shrimp on top of the avocados and garnish with more minced chives. Serve immediately.