Pickled Red Onions 

Ingredients 
1/2 cup red wine vinegar 
2 tablespoons sugar 
1 tablespoon Kosher salt 
1 large red onion, thinly sliced 
Black peppercorns, optional 
Mustard seeds, optional 
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Directions 
In a mason jar, combine the vinegar, sugar, and salt, cover and shake to combine. Add the onions, cover, and shake. Refrigerate for at least 1 hour before using. Drain onions before using. 

Notes
- You could make this up to 2 weeks in advance.