Using a mandoline or knife, slice peppers and onions thinly. For the peppers, you can choose to keep or remove the seeds. Keeping the seeds will add more heat to the end result.
In a small sauce pan, combine vinegar, water, garlic, salt, and sugar and bring to a soft boil. Add jalapenos and onions, stir once, and remove from heat. Let sit for about 12 minutes in the hot liquid.
Use on sandwiches, subs, hot dogs, tacos, quesadillas, and more! If you have any leftovers, let the onions and peppers cool and store in an airtight container with the liquid. Enjoy!