In a 1-quart glass jar, stir together the vinegar, cloves, garlic, sugar, salt, bay leaf, and cinnamon stick until the sugar dissolves.
Place red onions 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 until they are fully submerged.
Seal the jar and let the red onions pickle, refrigerated, for at least one hour before using, preferably overnight. Store for up to 3 weeks. **Makes 2 cups