Notes: Spring onions are freshly dug, small-bulbed onions with green stems still attached. They make for tender, mild pickled onions. Look for them in good produce markets or at farmers' markets. If they are unavailable, use pearl onions. You can make the onions up to 2 days ahead; cool, cover, and chill with liquid. Strain and bring to room temperature before serving.
1 1/2 cups distilled white vinegar
1 tablespoon kosher salt
1 tablespoon sugar
8 whole cloves
1 teaspoon hot chili flakes
1 dried bay leaf
12 ounces spring or pearl onions (about 30, 1 to 1 1/2 in. wide; trim any greens before weighing), rinsed (see notes)
How to Make It
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.
Meanwhile, peel onions and pull off any thin membrane under peel. Trim root ends.
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.