In medium sized saucepan, combine water, vinegar, salt, and sugar; over medium high heat, bring to a boil or until sugar and salt is dissolved, stirring occasionally. Set aside to cool slightly.
Place garlic clove, mustard seeds, peppercorns, and red pepper flakes at the bottom of a large jar (quart size jar). Arrange asparagus spears vertically in jar and tuck dill sprigs into the jar.
Pour the brine into the jar, covering the asparagus completely.
Cover jar and place in the fridge at least overnight but preferably a week before eating.
Store the jar of pickled asparagus in the refrigerator for up to 3 months.
Notes
Nutrition information includes brine which isn't consumed so don't be alarmed by the sodium content.
Leave out the garlic, dill, or red pepper flakes, or increase the amount used, whatever you like.
Use pickling spice instead, or make your own combination.
Add other vegetables to the jar such as sliced onion or sliced jalapeño pepper.