01 -
Wash the peppers, remove the stems, and slice into rings about a quarter inch thick. Peel the onion and slice it into thin half-moons.
02 -
Combine the vinegar, water, sugar, salt, smashed garlic, peppercorns, mustard seeds, and red pepper flakes in a saucepan. Warm over medium heat, stirring, until the sugar and salt fully dissolve. Do not let it boil.
03 -
Divide the peppers and onions between two clean pint jars, packing them fairly tight but not crushed. Tuck the garlic pieces down among the vegetables.
04 -
Ladle the warm brine over the vegetables until they are fully submerged, leaving about half an inch of headspace. Tap the jars gently to release air bubbles and seal the lids.
05 -
Let the jars cool to room temperature, then refrigerate. They are edible after one hour and reach peak flavor after twenty-four hours.