1. In a 1 1/2- to 2-quart pan over medium heat, combine vinegar, salt, sugar, cloves, chili flakes, bay leaf, and 1 1/2 cups water. Bring to a simmer and cook for 5 minutes.
2. Meanwhile, peel onions and pull off any thin membrane under peel. Trim root ends.
3. Add onions to vinegar mixture and simmer, stirring occasionally, until just tender when pierced, 10 to 15 minutes. Remove from heat and let cool to room temperature, about 1 hour. Using a slotted spoon, transfer to a serving bowl; discard liquid.