Snap the ends of the asparagus and pack them inside the glass jar. If any of the spears stick out of the top of the jar, trim off more of the ends. Add the garlic cloves, red pepper flakes, and dried dill to the jar.
In a small saucepan, combine the water, vinegar, salt, and sugar over medium high heat. Bring to a boil, stir until the sugar dissolves, remove from heat, and carefully pour the brine over the asparagus spears. Leave at least ¼" at the top of the jar to avoid overfilling with brine.
Let them cool for approximately 30 minutes at room temperature before sealing with the jar lid and transferring to the fridge. Refrigerate for at least 24 hours before eating. These taste even better after 3 or more days of refrigeration.
Notes
The asparagus will change color as it sits in the brine. The tips may become slightly pink or purple, and the garlic may turn blue. This is perfectly safe, and the asparagus is OK to eat.
If your asparagus is thick and wide, try alternating the spears (tip-side up vs. tip-side down) to fit them into the jar more easily.
Please note the sodium content in each serving is likely a lot less than listed here, since the nutrition calculations include the entire brine, which most people do not consume.