Peel jicama by cutting a thin slice off the top and bottom and paring sides with a small knife. Cut into 1/4-inch-thick slices, then cut slices into thin strips. Set aside.
Cut avocados in half lengthwise. Insert the blade of a heavy knife in pit, twist to remove, and discard. Peel avocado halves. Squeeze lemon juice over avocados to prevent browning; set aside.
Whisk together vinegar, oil, and salt and pepper to taste.
Make a bed of jicama on four plates, and top each with an avocado half. Season lightly with salt and pepper, sprinkle with onion, and pour dressing into avocado cavities. The salad will "dress" itself when you cut into avocados.