Trim the cut end of the asparagus spears, and cut into 3 inch lengths. Place them in a large bowl with 1/3 cup salt, and cover with water, let stand for 2 hours. Drain and rinse under cool water and pat dry.
Sterilize two pint size wide mouth jars in simmering water for 5 minutes.
In a saucepan over medium heat, combine the vinegar, sugar, 1 tea. salt, mustard seed, dill seed, and onion rings. Bring to a boil and boil for 1 minute.
Pack the asparagus spears, (tip end up) in the hot jars leaving 1/2 of head space from the rim. Tuck one dill sprig in each jar and sprinkle in 1/4 tea. of red pepper flakes. Pour hot pickling liquid into the jars filling to within 1/4 inch of the rim. Wipe rim with a clean dry cloth seal with lid and process in a boiling water bath for 10 minutes.
Cool 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.