Wear gloves to avoid irritation. Slice the jalapeños into thin, even rings using a sharp knife or mandoline. Discard the stems but keep the seeds if you like extra heat.

In a large pot or saucepan, combine the sugar, apple cider vinegar, turmeric, celery seed, mustard seeds, and cayenne pepper. Stir well to mix.

Bring the mixture to a boil over medium heat, then reduce the heat to low. Simmer for about 5 minutes, stirring occasionally, until the sugar is fully dissolved and the syrup is slightly thickened.

Add the sliced jalapeños to the pot, stirring them into the syrup. Bring the mixture back to a boil, then reduce the heat and simmer for 4-5 minutes. The jalapeños should be slightly softened but still retain their shape.

Using a slotted spoon, transfer the jalapeños to sterilized canning jars, packing them in tightly. Leave about 1/4 inch of headspace at the top of each jar.

Pour the hot syrup over the jalapeños, ensuring they are fully covered. If needed, use a canning funnel to avoid spills. Remove any air bubbles by gently tapping the jars or using a non-metallic utensil.

Wipe the rims of the jars with a clean, damp cloth, then place the lids on top and screw on the bands until fingertip-tight.

If you plan to store the cowboy candy for an extended period, process the jars in a boiling water bath for 10 minutes. Use tongs or a jar lifter to carefully lower the jars into the canner, ensuring they are fully submerged.

After processing, remove the jars and let them cool on a towel or wire rack. Listen for the "ping" of the lids sealing as they cool. Any unsealed jars should be refrigerated and used first.

Allow the jars to cool completely at room temperature. Store in a cool, dark place for at least 2 weeks before opening to allow the flavors to develop. Once opened, store in the refrigerator.