Pack sliced onion, pickling cucumbers, and serrano peppers in heat proof container, top off with one teaspoon dill.
Crush mustard seeds and peppercorns together using mortar and pestle or plastic bag and mallet.
Place crushed spices into a saucepan along with vinegar, water, sugar, and salt.
Bring mixture to a boil. When sugar dissolves, ladle mixture evenly over contents in the container. Make sure all cucumbers are submerged, leave uncovered on the counter for 30 mins. After brine has cooled, cover container with tight fitting lid, and place in refrigerator for 21 hours.
Pickles will keep nice and crunchy for one week covered in refrigerator.