Add vinegar, 1/4 cup of water, sugar and salt to a small sauce pan and bring to a boil. Remove from heat and cool while you prep the onion and jalapeno.
Halve the onion and thinly slice the red onion and jalapeno.
In a medium bowl, combine the onion, jalapeno, and peppercorns. Pour the vinegar over, cover, and refrigerate. Let sit for 4-24 hours before serving.