At the bottom of each jar, put 1/2 tsp. crushed recipe pepper flakes, 1 tsp. whole mustard seeds, 1 tsp. whole black peppercorns, and 2 cloves garlic.
Wash asparagus and cut to the size of the jar. Pack tightly.
Combine water, vinegar, and salt. Heat to boiling.
Cover asparagus with brine, leaving 1/2" headspace.
Fill water bath canner halfway with water. Place the canner on your largest burner, and turn on the heat so the water is very hot but not boiling. Place your filled jars into the canner's rack.
Lower the rack with the filled jars into the canner. Add simmering water if needed to cover the jars by 1 to 2 inches. Turn the heat to medium-high, place the lid on the canner, and bring the water to a full rolling boil.
Process quarts for 10 minutes at 0 to 1,000 feet elevation; 15 minutes at 1,001 to 6,000 feet elevation; 20 minutes above 6,000 feet.