In a small saucepan, combine water, vinegar, salt, sugar, alum, mustard seeds, peppercorns, and dill seeds. Bring to a boil over medium heat, stirring until salt and sugar dissolve. Remove from heat.
Slice serrano peppers to the desired thickness and transfer them to a sterilized mason jar. Add thinly sliced garlic cloves and shallot.
Pour pickle brine into the jar. If the contents are not fully submerged, you can top it off with additional water. Seal the jar and give it a good shake, then refrigerate at least 1 hour before consuming. For best results, I recommend pickling for 1-3 days.
Notes
Feel free to leave serrano peppers whole or to cut them into halves instead of coins.
Make sure you wash your hands after slicing the serranos! Better yet, you may even want to use gloves so you don't get pepper juice in any microcuts on your hands.
This recipe yields 16oz of pickled serranos and requires about 10 serranos. If you're trying to use up a larger quantity of peppers, you can double, triple, or even quadruple the batch.
Refrigerate pickled serrano peppers in a sealed glass mason jar or airtight container (glass is best).
If you want to extend their shelf life, give them as a gift, or store them in the pantry, you can follow the method used in this guide on how to can salsa.