In a 1-quart glass jar, stir together the vinegar, cloves, garlic, sugar, salt, bay leaf, and cinnamon stick until the sugar dissolves.
Place the onion slices in a mesh strainer set over the sink. Slowly pour the boiling water over the onions. Using tongs, immediately transfer the hot onions to the vinegar mixture. Gently press the onions down to fully submerge.
Seal the jar and let the onions pickle, refrigerated, for at least 1 hour before using, preferably overnight. Store refrigerated for up to 3 weeks.