Slice the Jalapenos - thin.
Boil the water, vinegar, garlic, salt and sugar together in a medium pot. Add jalapeno slices, stir, and remove from heat. Boiling the jalapenos will only make them soggy so you should remove them from the heat ASAP. The residual heat that lingers after boiling is all that is needed.
Let sit for about 10 - 15 minutes before placing in mason jars, add the diced red onion just before sealing the jars. Leave about 1/8 - 1/4 inch head space in each jar.
Refrigerated they will keep for 2 -3 months