Cut avocado in half lengthwise.
Remove pit and peel.
Cut each half into thin slices, lengthwise.
Spread avocado slices on serving plate in two semicircles or arcs, with slices overlapping.
Scatter onion slices over top of avocado slices.
Sprinkle liberally with salt and several grinds of pepper.
Drizzle oil and vinegar over slices.