Slice the round part of the onion thinly.

Soak the sliced onion in cold water for 5–10 minutes to remove its sharpness.

Chop 1/4 onion into blender-friendly pieces.

Add chopped onion, mayonnaise, vinegar, sugar, salt, and pepper to the blender.

Blend the mixture until smooth.

Transfer the sauce to a bowl and refrigerate for a chilled serving.

Drain the soaked onion slices and remove excess water.

Place the onion slices in a serving dish.