Wash the Brussels sprouts carefully, remove any damaged outer leaves, and trim the stems before cutting larger sprouts in half.
Bring a pot of water to a boil and blanch the Brussels sprouts for about two minutes before cooling them quickly in ice water.
Add garlic, dill, mustard seeds, and red pepper flakes into clean jars to create the seasoning base.
Pack the cooled Brussels sprouts tightly into the jars while leaving enough room for the pickling liquid.
Heat vinegar, water, salt, and sugar together until the salt and sugar dissolve completely.
Pour the warm brine into the jars, making sure all Brussels sprouts are covered beneath the surface.
Refrigerate the jars for several days before serving so the flavors fully develop and the sprouts become deliciously tangy.