In a large pot or bowl, layer sliced squash and sliced onions and sprinkle each layer generously with kosher salt and let sit for 1 hour. Drain well.
In a large non-reactive pot combine vinegar, sugar, pickling spice and crushed red pepper and bring to a boil. Add squash and onions and return to a boil.
Divide vegetables between sterilized jars and ladle enough pickling liquid over vegetables to come within 1/2–inch from top of jar. Wipe rims of jars clean, cover with lids and screw tops in place. Process jars in a hot water bath for 10 minutes. Remove jars from water bath and cool. Jars should seal as they cool. Any jars that do not seal should be refrigerated and consumed within 2 weeks.
Yield: approximately 4 pints of pickles.