The recipe below is very basic to keep the flavor profile more versatile. If you wanted to play around, you could add some whole peppercorns, garlic clove(s), fresh herbs, or the like.
Quick Pickled Red Onion
From Sprouted Kitchen
Yield: about 1 cup (the photos above show a single batch in a pint sized mason jar)
Ingredients
1/2 cup red wine vinegar
1 tsp. kosher salt
1 tsp. sugar
1 small red onion or 1/2 a medium one, sliced vertically
Directions
Add vinegar, salt, and sugar to a jar or other microwave safe glass dish. Warm it in the microwave for just a bit (30-60 seconds ought to do it) to help the sugar and salt dissolve. Stir. Add your red onion slices and press down into the liquid. If needed, add a splash of water and/or a bit more vinegar so the onion is mostly covered. Let sit for at least 30 minutes before eating. I find it helps to put the lid on the jar and turn the jar a couple times during those 30 minutes.
Store in the fridge. I like to eat mine within 2 weeks or so, but I've seen nearly identical recipes that say they'll last for over a month.
Tip
To vertically slice an onion, you basically are slicing from pole to pole instead of around the hemisphere. Slice off the root end and stem end (the poles), then cut the onion in half from pole to pole. With an onion half flat side down on the cutting board, make thin slices, again from pole to pole. Here's a video from Cooking Light.