Slice red onion crosswise into rings 1/4 inch thick.

In saucepan over low heat, combine vinegar, wine, sugar, mustard seeds, peppercorns, red pepper flakes and salt.

Stir until sugar and salt are dissolved.

Add onions and bring liquid to a boil.

Reduce heat and simmer for 5 minutes.

Let cool completely before serving.

Transfer onions and liquid to mason jar and refrigerate.

Bring to room temperature before serving.