Step 1

Slice the red onion as thin as possible. Use a mandolin for evenly sliced onions.

Step 2

Add the sliced red onions to a glass jar, such as a 16-ounce mason jar. Even a bowl will work or container with a lid, glass preferred.

Step 3

In a small saucepan, combine the white vinegar, water, sugar, salt and pepper flakes. Bring the mixture to a simmer over medium heat, then immediately remove from heat.

Step 4

Pour the pickling liquid over the onions in the jar. Use a butter knife to press the onions down into the liquid, making sure they are immersed completely. Let them sit at room temperature, uncovered, for 30 minutes, then cover and refrigerate for at least an hour before serving. They will have a stronger flavor after at least 4 hours in the refrigerator.

Step 5

They will keep for approximately 2 weeks in the refrigerator.