Bring the water to a boil in a medium saucepan with the garlic clove.

Place the sliced onions in a heatproof bowl and pour the boiling water over the onions. let sit until slightly softened, then drain in a colander. Transfer onion slices and garlic clove to a glass jar or other nonreactive container and pour the vinegar over the onion slices. Add the salt and sugar and stir until dissolved. Set onions aside to marinate for at least 30 minutes and up to several days.

Taste and adjust by adding salt and/or sugar to taste. Store in a covered nonreactive container in the refrigerator for up  to 2 weeks.