Wash and dry the fresh jalapeño peppers. Using a sharp knife, cut into ¼-inch pepper slices. Remove seeds if you want a milder batch.
In a large pot, combine apple cider vinegar, white sugar, garlic powder, celery seed, turmeric, and mustard seed. Stir over medium heat until the sugar dissolves. Bring to a full rolling boil, then reduce heat slightly.
Add the sliced peppers to the simmering vinegar mixture. Stir and let them cook for 5 minutes until the texture of the peppers turns glossy and softened.
Using a slotted spoon, transfer the candied jalapeños into sterile canning jars (half pints or pint jars). Pack peppers down with a butter knife or chopstick so there are no air pockets. Leave ¼ inch of the upper rim of the jar clear.
Boil the remainder liquid that is left in the pot on high for an additional 5 minutes. This will help to thicken the sauce up. Then, ladle the hot syrup over the peppers, filling to the top of the jar but still leaving that ¼-inch headspace. Use a butter knife again to release air bubbles.