Trim the cut end of asparagus spears; slice them into 3-inch lengths. Place in a large bowl with 1/3 cup salt. Pour in water to cover asparagus; let stand for 2 hours. Drain and rinse under cool water, and pat dry.

Sterilize two pint-sized wide mouth jars in simmering water for 5 minutes. In a saucepan over medium heat, combine vinegar, sugar, onion, dill seed, 1 teaspoon salt, and mustard seed. Bring to a boil and simmer for one minute.

Pack asparagus spears, tips up, in the hot jars leaving 1/2 inch of space from the rim. Tuck one dill sprig into each jar; sprinkle in 1/4 teaspoon of red pepper flakes. Pour hot pickling liquid into the jars, filling to within 1/4 inch of the rim. Wipe rims with a clean damp cloth and seal with lids.

Place a rack in the bottom of a large stockpot and fill halfway with water. Bring to a boil and lower jars 2 inches apart into the boiling water using a holder. Pour in more boiling water to cover jars by at least 1 inch. Bring to a rolling boil, cover, and process for 10 minutes.

Cool jars to room temperature. Check seals when cool by pressing the center of the lid. It should not move. Label and date; store in a cool dark place. If any jars have not sealed properly, refrigerate and eat within two weeks.