Use these to top our Blue Cheese, Potato, and Green-Apple Danish Sandwiches, as well as our Salami Danish Sandwiches.
Stir vinegar, 1/4 cup water, the sugar, bay leaf, peppercorns, and salt in a saucepan. Bring to a boil. Add onion; return to a boil. Cook, stirring, 30 seconds. Remove from heat; let cool completely.
Onion slices can be refrigerated in their liquid in an airtight container up to 2 weeks.