Using a sharp knife or a mandolin, slice your onion halves in a parallel direction to produce long, 1/4-1/2’’ strips.
Bring water to a boil. Place the onions in a colander and pour 1 cup of boiling water over them. This tones down the strong onion flavor and par cooks them.
Transfer the remaining water to a small saucepan, along with the vinegars, sugar, and salt. Bring to a boil, then simmer until sugar dissolves—1 minute. Taste and adjust seasonings if needed, then remove from heat.
Place onions and peppercorns in a jar and pour warm vinegar over all. Allow to sit at least 45 minutes before serving.
Allow onions to come to room temperature before storing in the refrigerator.