Sterilize your jars by boiling them in water for 10 minutes. Keep them warm until ready to use.
In a stainless steel or enamel saucepan, combine the water, vinegar, sugar, kosher salt, black peppercorns, mustard seeds, fennel seeds, and crushed red pepper flakes. Bring the mixture to a boil, stirring until the sugar and salt dissolve.
Add the garlic to the boiling mixture and cook for 1 minute.
Using a slotted spoon, divide the garlic between the sterilized jars, leaving ½ inch of headspace.
Add a sprig of fresh dill to each jar.
Pour the hot pickling liquid into the jars, covering the garlic and leaving ½ inch of headspace.
Remove any air bubbles, wipe the rims of the jars with a clean, damp cloth, and close the jars tightly.
Store the jars in the refrigerator for at least 3 weeks before consuming. The pickled garlic will last 4 months in the refrigerator.