Slice 10 to 11 medium jalapeno peppers.
Remove all of the white membrane and about half of the seeds from each slice.
In a large saucepan over medium heat, combine vinegar, sugar, salt, peppercorns, garlic and mustard seed. Bring mixture to a full boil.
Add peppers. Stir to coat and remove from heat to maintain crispness.
Ladle peppers into clean, warm containers, covering the pepper rings with liquid.
Cover tightly and let stand at room temperature for 1 hour to cool.
Transfer to the refrigerator and let stand for at least 3 days before eating. Pickled peppers may be stored in the refrigerator for up to 3 weeks. Leftover peppers may be frozen for longer storage.