The reviews I read said these were pretty tart, so I cut back just a bit on the red wine vinegar. This recipe is easy, fun to make and perfect for burgers or salads.

Bobby Flay's Pickled Onions
2 large red onions, peeled and thinly sliced (I used sweet onions)
1 tablespoon olive oil
3 cups red wine vinegar
1/4 cup olive oil
2 tablespoons sugar
6 whole black peppercorns
4 sprigs fresh thyme
Garnish: 2 tablespoons finely chopped thyme
Directions
Saute the onions over low heat in 1 tablespoon of olive oil until soft. Place the vinegar, 1/4 cup of olive oil, sugar, peppercorns and thyme in a large noncorrosive saucepan and bring to a boil. Add the onions to the hot liquid and simmer for 5 minutes. Remove the pot from the heat and allow the onions to steep for at least 2 hours or overnight. (if storing overnight, place in a bowl and cover.) When ready to serve, drain the liquid and top with thyme.
I am linking this to Foodie Friday and Full Plate Thursday.