Peel and trim small onions or shallots
Pack them tightly into a clean sterilized jar
Add spices if desired, such as peppercorns, mustard seeds, or bay leaf
Heat equal parts vinegar and water in a saucepan
Add salt and sugar to the hot liquid and stir until dissolved
Pour the hot brine over the onions until fully covered
Tap the jar gently to remove air bubbles
Seal the jar with a lid
Let the jar cool to room temperature
Refrigerate for at least 24 hours before eating
Store in the refrigerator and use within a few weeks